Android Chromoting: Remove exit-fullscreen button.
This removes the ancient Gingerbread icon button for exiting fullscreen
and implements swipe-to-exit-fullscreen instead (by using
IMMERSIVE instead of IMMERSIVE_STICKY). With this change, the screen
would occasionally flip-flop endlessly in and out of fullscreen mode,
so I've also refactored the code not to change the SysUiVisibility flag
in response to SysUiVisibility change notifications.
BUG=430328
Review URL: https://codereview.chromium.org/
1054273003
Cr-Commit-Position: refs/heads/master@{#323681}