Fixing crash when drawing accessibility focus indicator.

1. Added a lacking null check for the accessibility node
   info returned by an accessibility node provider. If
   the provider implementation is not correct this may
   happen.

2. Added clearing of the current accessibility focused
   node when the window focus changes. Now it is cleared
   in every case and if it happens that accessibility is
   enabled when the window gets focus, the accessibility
   focus will be properly set.

bug:6381296

Change-Id: Ieea1b07762745e6d932fc4ed4febfe77760b25b7