Store initial bouncer showing state.

This changelist keeps track of the initial bouncer showing state when
scrolling begins on the bouncer. This ensures the value does not change
mid scroll.

This changelist also cleans up session callbacks. onSessionEnd had
migrated to be a callback on the handler to a registered callback on
the session.

Bug: 223472045
Test: BouncerSwipeTouchHandlerTest DreamOverlayTouchMonitorTest
Change-Id: Ib2c295ce8f27d0f40f33039a71ec39b52ef96ddb