Address TouchMonitor memory leaks.

This changelist addresses TouchMonitor memory leaks by:
- Clearing TouchMonitor's reference to the gesture exclusion listener to
  prevent circular references.
- Removing lifecycle observers and lifecycle flows when the TouchMonitor
  is destroyed.
- Clearing references to touch sessions when no longer used.

Fixes: 338532926
Test: DreamOverlayServiceTest#testOnEndDream
Test: TouchMonitorTest#testDestroy_cleansUpLifecycleObserver
Flag: EXEMPT bugfix
Change-Id: I6979c8cbcd8fa9baebf962927c93325eb6eb688c