Notification muting conditional to exclusive audio focus request

Summary of feature:
Do not mute notifications when speech recognition recording is
  active, but when an app has requested audio focus with
  AudioManager.AUDIOFOCUS_GAIN_TRANSIENT_EXCLUSIVE.

Implementation:
Move definition of AUDIOFOCUS_NONE to AudioManager where other
  audio focus codes are defined.
Add support for querying the current audio focus type.
When audio focus is requested as GAIN_TRANSIENT_EXCLUSIVE, make
  the corresponding loss by LOSS_TRANSIENT.
Before playing a notification, check whether GAIN_TRANSIENT_EXCLUSIVE
  has been requested.

Bug 8251963

Change-Id: I41edc77326b70639d2fdcb4642c53109995b72a8

media/java/android/media/MediaFocusControl.java

@@ -424,6 +424,15 @@ public class MediaFocusControl implements OnFinished {
         }
     }
 
+    protected int getCurrentAudioFocus() {
+        synchronized(mAudioFocusLock) {
+            if (mFocusStack.empty()) {
+                return AudioManager.AUDIOFOCUS_NONE;
+            } else {
+                return mFocusStack.peek().getGainRequest();
+            }
+        }
+    }
 
     /** @see AudioManager#requestAudioFocus(AudioManager.OnAudioFocusChangeListener, int, int)  */
     protected int requestAudioFocus(int mainStreamType, int focusChangeHint, IBinder cb,