Use immersive mode pref for full edge exclusion from System Gestures

Current approach won't work because Android enforces a 200dp vertical
limit, unless app is in sticky immersive mode.

app/src/main/java/com/gaurav/avnc/ui/vnc/VncActivity.kt

@@ -256,7 +256,7 @@ class VncActivity : AppCompatActivity() {
                 if (left < 0) rect.offset(-left, 0)
                 if (right > root.width) rect.offset(-(right - root.width), 0)
 
-                if (fullscreenMode) {
+                if (immersiveMode) {
                     rect.top = 0
                     rect.bottom = root.height
                 }