[audio] Fix AudioRecordingMonitorImpl leak

Ensure that we don't leak resources if all callbacks aren't unregistered
prior to release.

Remove handler thread to align with avoiding handler trampolines when
consuming executors, which also avoids the additional thread cost and
leak risk.

Some minor refactoring/cleanup as part of these changes.

Bug: 432245274
Flag: EXEMPT bugfix
Test: atest AudioRecordTest
Change-Id: I6a6a6964bcf78f9acc47a4eaebe45f39918011f2