Android: implement headphone detection thus enabling pause on unplug (FS#12097).

Listen to headphone plug events. There are currently two glitches with this:
- Android takes a while until it reports the unplug event, so there will be
  some delay until playback gets paused. This is an Android limitation.
- Rockbox debounces headphone state changes for one second. Therefore playback
  will shortly be routed to the speaker on unplug until Rockbox does the actual
  pause.

git-svn-id: svn://svn.rockbox.org/rockbox/trunk@29956 a1c6a512-1295-4272-9138-f99709370657

android/src/org/rockbox/RockboxService.java

@@ -64,10 +64,13 @@ public class RockboxService extends Service
     private static volatile boolean rockbox_running;
     private Activity current_activity = null;
     private IntentFilter itf;
+    private IntentFilter ifh;
     private BroadcastReceiver batt_monitor;
+    private BroadcastReceiver headphone_monitor;
     private RunForegroundManager fg_runner;
     private MediaButtonReceiver mMediaButtonReceiver;
     private int battery_level;
+    private int headphone_state;
     private ResultReceiver resultReceiver;
 
     public static final int RESULT_INVOKING_MAIN = 0;
@@ -339,6 +342,24 @@ public class RockboxService extends Service
         registerReceiver(batt_monitor, itf);
     }
 
+
+    private void initHeadphoneMonitor()
+    {
+        ifh = new IntentFilter(Intent.ACTION_HEADSET_PLUG);
+        headphone_monitor = new BroadcastReceiver()
+        {
+            @Override
+            public void onReceive(Context context, Intent intent)
+            {
+                int state = intent.getIntExtra("state", -1);
+                LOG("headphone state:" + state);
+                headphone_state = state;
+            }
+        };
+        registerReceiver(headphone_monitor, ifh);
+    }
+
+
     void startForeground()
     {
         fg_runner.startForeground();