Protect bubble gesture monitor against re-entrant calls

We were seeing an NPE when stopping bubble gesture monitor when stack
view was collapsing. The cause seems to be triggered by a re-entrant
call that originates from BatchedInputEventReceiver sending out all
unconsumed touch events when it is disposed.

Some of these unconsumed touch events can trigger another collapse call.
Which tries to stop the gesture monitor again. Since we clean up the
reference to input event receivers after dispose() call, the references
are still there. And this triggers another attempt to dispose them. This
now leads to an NPE and the input event receivers are already cleared
out.

Fixing this by ensuring we clear out the references to the input event
receivers before we call dispose on them. To protect against re-entrant
calls.

Bug: 432344114
Test: manual, check that bubbles input monitor stops (regular home gesture works) when:
- bubbles are expanded and collapsed
- bubbles are expanded and notif panel is expanded on top
Flag: EXEMPT, bugfix

Change-Id: I7e882892a4c1cdb4f8ed0fa252125032feeb1806