MessageStack: add quitting sentinel node

We use the timestamp of the quitting message to tell
DeliQueue whether it should remove all messages or just
remove messages past a given time ("removeAllFutureMessages").

Test: atest android.os.MessageStackTest
Bug: 421623328
Flag: EXEMPT new data structure isn't used yet; usages will be flagged.
Change-Id: I7caf36e9a5fc1efec1be7f733b4b2d7f2f176f92