Cannot click on partially visible views in touch exploration.

1. In touch exploration mode the system clicks in the center of the
   accessibility focus rectangle. However, if this rectangle is only
   partially shown on the window or on the screen the system may not
   be able to perform the click, if the accessibility focus center
   is not on the screen, or click on the wrong window, if the access
   focus center is outside of the window.

   This change clips the rectangle to the window bounds which and the
   display bounds. This will ensure no clicks are sent to the wrong
   window and no clicks are sent outside of the screen.

bug:7453839

Change-Id: I79f98971e7ebcbb391c37284467dc76076172c5f