MessageStack.updateFreelist(): return whether messages were removed

DeliQueue.removeSyncBarrier() needs to know if the barrier it
asked to remove actually got removed.

Also fix a null pointer deref of MessageStack.firstRemoved which
happens when we don't find any messages to remove.

Test: atest android.os.MessageStackTest
Bug: 421623328
Flag: EXEMPT new data structure isn't used yet; usages will be flagged.
Change-Id: I0dc917ee2d1056acd2f1cef4091ee3df1ee26e87