Actually prevent a repeated unregistration of mBatchAlarmReceiver.

Uses a AtomicReferences that is read and atomically set to null. Also moves it to the ScanNative inner class, and makes the rest of the ScanNative members final.

Note that there are other receivers in ScanManager that may be unregistered multiple times. In those cases the IllegalArgumentException is caught. We should consider changing that, because if cleanup is called multiple times there will be a bunch of errors logged which could cause issues to be mistriaged. But that is out of scope of this change.

Bug: 354303095
Fix: 354303095
Flag: EXEMPT, what isn't mechanical refactor is trivial
Test: atest ScanManagerTest, atest BumbleBluetoothTests
Change-Id: Ib3982c3a933459b77aa305f54517b527e55701c9