Properly flush output writer during server export

Fixes data not being written to exported file.
`use()` function is used to automatically close & flush the writer.
Reader used for import-file is updated too.

Also added tests for import/export.

app/src/androidTest/java/com/gaurav/avnc/ui/pref/ImportExportTest.kt

@@ -0,0 +1,99 @@
+/*
+ * Copyright (c) 2022  Gaurav Ujjwal.
+ *
+ * SPDX-License-Identifier:  GPL-3.0-or-later
+ *
+ * See COPYING.txt for more details.
+ */
+
+package com.gaurav.avnc.ui.pref
+
+import android.app.Activity
+import android.app.Instrumentation
+import android.content.Intent
+import androidx.core.net.toUri
+import androidx.test.espresso.Espresso.onView
+import androidx.test.espresso.intent.Intents
+import androidx.test.espresso.intent.matcher.IntentMatchers
+import androidx.test.espresso.matcher.ViewMatchers.withSubstring
+import androidx.test.espresso.matcher.ViewMatchers.withText
+import androidx.test.ext.junit.rules.ActivityScenarioRule
+import com.gaurav.avnc.*
+import com.gaurav.avnc.model.ServerProfile
+import com.gaurav.avnc.model.db.MainDb
+import com.gaurav.avnc.ui.prefs.PrefsActivity
+import com.gaurav.avnc.util.DeviceAuthPrompt
+import kotlinx.coroutines.runBlocking
+import org.junit.*
+import java.io.File
+
+class ImportExportTest {
+    @Rule
+    @JvmField
+    val activityRule = ActivityScenarioRule(PrefsActivity::class.java)
+
+    @Before
+    fun openImportExportScreen() {
+        onView(withText(R.string.pref_tools)).doClick()
+        onView(withText(R.string.pref_import_export)).doClick()
+        onView(withText(R.string.title_import)).checkIsDisplayed()
+        onView(withText(R.string.title_export)).checkIsDisplayed()
+    }
+
+    @Before
+    fun initIntents() = Intents.init()
+
+    @After
+    fun releaseIntents() = Intents.release()
+
+
+    @Test
+    fun exportTest() {
+        // This test requires an unlocked device
+        activityRule.scenario.onActivity { Assume.assumeFalse(DeviceAuthPrompt(it).canLaunch()) }
+
+        // Insert sample data
+        val sampleName = "Days of our Lives"
+        runBlocking { MainDb.getInstance(targetContext).serverProfileDao.insert(ServerProfile(name = sampleName)) }
+
+        // Setup export file
+        val file = File.createTempFile("avnc", "test")
+        Intents.intending(IntentMatchers.hasAction(Intent.ACTION_CREATE_DOCUMENT))
+                .respondWith(Instrumentation.ActivityResult(Activity.RESULT_OK, Intent().setData(file.toUri())))
+
+        // Export
+        onView(withText(R.string.title_export)).doClick()
+        onView(withText(R.string.msg_exported)).checkWillBeDisplayed()
+        onView(withSubstring("Error")).checkIsNotDisplayed()
+
+        // Verify exported data
+        instrumentation.waitForIdleSync()
+        val data = file.bufferedReader().readText()
+        Assert.assertTrue("Exported data: `$data` contains `$sampleName`", data.contains(sampleName))
+    }
+
+
+    @Test
+    fun importTest() {
+        val sampleName = "Joey Tribbiani"
+        val sampleJson = """{ "profiles": [{ "name": "$sampleName" }]}"""
+
+        // Setup import file
+        val file = File.createTempFile("avnc", "test")
+        file.bufferedWriter().use { it.write(sampleJson) }
+        Intents.intending(IntentMatchers.hasAction(Intent.ACTION_OPEN_DOCUMENT))
+                .respondWith(Instrumentation.ActivityResult(Activity.RESULT_OK, Intent().setData(file.toUri())))
+
+        // Import
+        onView(withText(R.string.title_delete_servers_before_import)).doClick()
+        onView(withText(R.string.title_import)).doClick()
+        onView(withText(R.string.msg_imported)).checkWillBeDisplayed()
+        onView(withSubstring("Error")).checkIsNotDisplayed()
+
+        // Verify imported data
+        instrumentation.waitForIdleSync()
+        val profiles = runBlocking { MainDb.getInstance(targetContext).serverProfileDao.getList() }
+        Assert.assertEquals(1, profiles.size)
+        Assert.assertEquals(sampleName, profiles.first().name)
+    }
+}

app/src/main/java/com/gaurav/avnc/viewmodel/PrefsViewModel.kt

@@ -63,9 +63,9 @@ class PrefsViewModel(app: Application) : BaseViewModel(app) {
                 val json = serializer.encodeToString(data)
 
                 // Write out
-                app.contentResolver.openOutputStream(uri).use {
-                    it?.writer()?.write(json) ?: throw IOException("Unable to write the file.")
-                }
+                app.contentResolver.openOutputStream(uri)?.use { stream ->
+                    stream.writer().use { it.write(json) }
+                } ?: throw IOException("Unable to write the file.")
 
             }.let {
                 importExportError.postValue(it.exceptionOrNull()?.message)
@@ -82,9 +82,9 @@ class PrefsViewModel(app: Application) : BaseViewModel(app) {
         asyncIO {
             runCatching {
 
-                val json = app.contentResolver.openInputStream(uri).use {
-                    it?.reader()?.readText() ?: throw IOException("Unable to read the file.")
-                }
+                val json = app.contentResolver.openInputStream(uri)?.use { stream ->
+                    stream.reader().use { it.readText() }
+                } ?: throw IOException("Unable to read the file.")
 
                 // Deserialize
                 val data = serializer.decodeFromString<Container>(json)