Updates Assistant gesture regions when no longer tracking multiple.

This should prevent states where Assistant triggers from
the vertical-center of the screen in portrait (see bug).

Also fleshes out OrientationTouchTransformerTests and
adds some new ones that fail without this change:
 - enableMultipleRegions_assistantTriggersInMostRecent
 - enableMultipleRegions_assistantTriggersInCurrentOrientationAfterDisable

Fixes: 158686674
Change-Id: I6d045a485f62e4010e9e3d00805a50fdd953a2fc