Fix crash when selecting edge swipe settings

When selecting a setting from the discovery panel
the setting is updated asynchronously, this
results in a view inviladitaion being called on the
wrong thread.
Changed it to Post it to the UI thread when available.

Change-Id: I08ad30585da9b9b8a89dec69dc1cb216d0a23cd6
diff --git a/src/com/android/browser/EdgeSwipeSettings.java b/src/com/android/browser/EdgeSwipeSettings.java
index dd6da87..105434b 100644
--- a/src/com/android/browser/EdgeSwipeSettings.java
+++ b/src/com/android/browser/EdgeSwipeSettings.java
@@ -169,7 +169,7 @@
         mStationaryView.setVisibility(View.GONE);
         mSlidingViewShadow.setVisibility(View.GONE);
         mSettingsView.setVisibility(View.GONE);
-        mViewGroup.invalidate();
+        mViewGroup.postInvalidate();
     }
 
     private void goDormant() {