Cancel all pending anims when tearing down.

When I was trying to reproduce a TH failure in DisplayContentTests I ran
into a different failure that a SurfaceControl (an anim leash) is used
after being destroyed. Therefore cancel all pending anims before
destroying all SurfaceControls in WMSRule to avoid that.

Bug: None
Test: Tests pass.
Change-Id: Iec978605fa5a8d7dbb0f8afed7cd2e6463454d04