Ensure onAcceptFailed is processed on the handler thread.

onAcceptFailed was invoked from an external thread,
leading to concurrent access to mServerSockets.
This resulted in a race condition and potential NPE.

This change dispatches onAcceptFailed handling to
the mHandlerThread to prevent race conditions.

Bug: 396306579
Bug: 380817240
Test: atest BluetoothInstrumentationTests
Change-Id: Id13940f6ee3ae33c88185cc2e011ac46212a1baf