Fix StringIndexOutOfBoundsException in AccessibilityUtils

The string splitter is a sigleton but the access is not thread-safe. So
now it's changed to be a local instance to avoid threading problems.

Also removed a few unused test classes as clean up.

Change-Id: Ia2b3f3058fcbb0b28afff4e3d1b29184072bf45e
Fixes: 110255579
Test: robotests