Add MessageStack, a Treiber stack of Messages

MessageStack allows for concurrent adding and removing of messages, and
hooks into MessageHeap for ordering of messages. This is intended for
use in the new message queue implementation.

Test: atest android.os.MessageStackTest
Bug: 421623328
Flag: EXEMPT new data structure isn't used yet; usages will be flagged.
Change-Id: I019d427ce9df4ca9dbb761375f9854ed9a595fa7