Use a stack node to indicate that MessageQueue is quitting

This makes it easier to reason about our quitting logic.  In particular,
it's very easy to determine that enqueue won't race with teardown.

We further simplify things by having next() remove messages during
teardown for us.

Test: atest MessageQueueTest
Test: atest LooperTest
Test: atest HandlerTest
Test: atest HandlerThreadTest
Bug: 411488376
Bug: 413185945
Flag: build.RELEASE_PACKAGE_MESSAGEQUEUE_IMPLEMENTATION
Change-Id: I3aac55ad987ff94a71b5f07e81070eb77809abe7