Add Touch Initiation Regions.

This changelist adds touch initiation regions for DreamTouchHandler.
Regions allow handlers to specify where the initial input event must
occur in for a session directed at it to start. In turn, this allows
for an accurate understanding of active sessions at any given time as
the touch manager can preemptively prune non participating sessions.

This changelist also updates the BouncerSwipeTouchHandler to now
specify a region rather than check the first event. A new method has
been added to touch session to query the number of active sessions.

Bug: 214292772
Test: atest DreamOverlayTouchMonitorTest BouncerSwipeTouchHandlerTest
Change-Id: Iaa5fe71d0375c8ee8e74e12eeffdcba3f51187ec