Fix DreamOverlayService memory leaks.

This changelist addresses a various points where references retain the
DreamOverlayService and/or its child members after the service is
destroyed.

Fixes: 5897130
Test: atest TouchMonitorTest#testDestroy_cleansUpHandler
Test: atest DreamOverlayContainerViewControllerTest#destroy_cleansUpState
Test: atest DreamOverlayServiceTest#testResetCurrentOverlayWhenConnectedToNewDream
Flag: EXEMPT bugfix
Change-Id: I5e9ea0e27f7219d9252b1480bbbc880851d2a496