Fix handling multiple staged rollback sessions ready signal

After we stage a session for rollback, we need to wait for the
session to be ready and then reboot. To ensure we don't miss the
ready signal, we register a BroadcastReceiver to listen to session changes
and also check for session ready right away. This caused a race condition
where we may handle post-ready twice. This caused a crash because we
attempt to unregister the receiver twice, it's also a problem because
we could log the same event twice.

Now, we store the rollback ids we are about to handle and ensure we never
handle post-ready more than once.

Test: Manual test && atest StagedRollbackTest
Bug: 132866890
Change-Id: I5187ff20fb83b29f7a00a28bf6ad8105ca4f0067