add accessibility descriptions/events

    Bug: 4979091
    added content descriptions to the image buttons
    send event for screen transition

Change-Id: I403fe8504cace4aa84a5b1c6ca18f6c10fc9be34
diff --git a/src/com/android/browser/NavScreen.java b/src/com/android/browser/NavScreen.java
index a361136..9c12ccf 100644
--- a/src/com/android/browser/NavScreen.java
+++ b/src/com/android/browser/NavScreen.java
@@ -111,6 +111,8 @@
 
     private void init() {
         LayoutInflater.from(mContext).inflate(R.layout.nav_screen, this);
+        setContentDescription(mContext.getResources().getString(
+                R.string.accessibility_transition_navscreen));
         mBookmarks = (ImageButton) findViewById(R.id.bookmarks);
         mNewTab = (ImageButton) findViewById(R.id.newtab);
         mNewIncognito = (ImageButton) findViewById(R.id.newincognito);