Ensure TouchSessions are always removed.

Gesture events normally drive TouchSession cleanup with up/cancel
events triggering the removal of any active sessions. However, it
is possible for the touch monitor to be stopped before this point.
This changelist ensure that the sessions are properly cleaned up
when monitoring stops before the expected touch events.

Test: atest DreamOverlayTouchMonitorTest#testOnRemovedCallbackOnStopMonitor
Fixes: 245441667
Change-Id: Id6817e726058b0a2738cbb7c163d2efa5eb3875e