[Android] Handle audio focus changes while in a conference

android/sdk/src/main/java/org/jitsi/meet/sdk/AudioModeModule.java

  * Before a call has started and after it has ended the
  * {@code AudioModeModule.DEFAULT} mode should be used.
  */
-class AudioModeModule extends ReactContextBaseJavaModule {
+class AudioModeModule extends ReactContextBaseJavaModule implements AudioManager.OnAudioFocusChangeListener {
     /**
      * Constants representing the audio mode.
      * - DEFAULT: Used before and after every call. It represents the default
      */
     static final String TAG = MODULE_NAME;
 
+    /**
+     * Indicator that we have lost audio focus.
+     */
+    private boolean audioFocusLost = false;
+
     /**
      * {@link AudioManager} instance used to interact with the Android audio
      * subsystem.
         });
     }
 
+    /**
+     * {@link AudioManager.OnAudioFocusChangeListener} interface method. Called
+     * when the audio focus of the system is updated.
+     *
+     * @param focusChange - The type of focus change.
+     */
+    @Override
+    public void onAudioFocusChange(int focusChange) {
+        switch (focusChange) {
+        case AudioManager.AUDIOFOCUS_GAIN: {
+            Log.d(TAG, "Audio focus gained");
+            // Some other application potentially stole our audio focus
+            // temporarily. Restore our mode.
+            if (audioFocusLost) {
+                updateAudioRoute(mode);
+            }
+            audioFocusLost = false;
+            break;
+        }
+        case AudioManager.AUDIOFOCUS_LOSS:
+        case AudioManager.AUDIOFOCUS_LOSS_TRANSIENT:
+        case AudioManager.AUDIOFOCUS_LOSS_TRANSIENT_CAN_DUCK: {
+            Log.d(TAG, "Audio focus lost");
+            audioFocusLost = true;
+            break;
+        }
+
+        }
+    }
+
     /**
      * Sets the user selected audio device as the active audio device.
      *
         Log.d(TAG, "Update audio route for mode: " + mode);
 
         if (mode == DEFAULT) {
+            audioFocusLost = false;
             audioManager.setMode(AudioManager.MODE_NORMAL);
-            audioManager.abandonAudioFocus(null);
+            audioManager.abandonAudioFocus(this);
             audioManager.setSpeakerphoneOn(false);
             setBluetoothAudioRoute(false);
             selectedDevice = null;
         audioManager.setMicrophoneMute(false);
 
         if (audioManager.requestAudioFocus(
-                    null,
+                    this,
                     AudioManager.STREAM_VOICE_CALL,
                     AudioManager.AUDIOFOCUS_GAIN)
                 == AudioManager.AUDIOFOCUS_REQUEST_FAILED) {