Fixing some flakiness in the CTS accessibility tests.

1. Imporved synchronization while waiting for async
   accessibility events.

2. Updated the gloabl action tests to avoid leading
   the device in a state that causes subsequent tests
   to fail.

bug:6301007
bug:6339712
bug:6467998
bug:6468754
bug:6339616
bug:6339713
bug:6468136

Change-Id: I1657f6089ef07c40051b69a0d86d09dfdb4c2307