Stop screen sharing when exiting MediaProjectionDemo.

The existing implementation will leave screen sharing running after
stop, causing the VirtualDisplay to not be destroyed. This change
makes sure any screen sharing ends on stop.

This change also addresses the the ToggleButton's state being saved.

Test: manual
Bug: 26221019
Change-Id: I51ac8bf09932a83100776c919146eed8c28f35b3

samples/ApiDemos/src/com/example/android/apis/media/projection/MediaProjectionDemo.java

@@ -88,6 +88,13 @@ public class MediaProjectionDemo extends Activity {
         s.setSelection(0);
 
         mToggle = (ToggleButton) findViewById(R.id.screen_sharing_toggle);
+        mToggle.setSaveEnabled(false);
+    }
+
+    @Override
+    protected void onStop() {
+        stopScreenSharing();
+        super.onStop();
     }
 
     @Override
@@ -140,6 +147,7 @@ public class MediaProjectionDemo extends Activity {
         if (mToggle.isChecked()) {
             mToggle.setChecked(false);
         }
+
         mScreenSharing = false;
         if (mVirtualDisplay != null) {
             mVirtualDisplay.release();