AIDL: Add unit tests to external device disconnection

Add tests for the code added in aosp/3005114.
Fix a minor issue found. It can lead to accumulating
entries in `mFwkPatches` for "implicit" patches.
This should rarely occur in practice as the framework
usually creates explicit patches for external devices.

Bug: 327449747
Test: atest CoreAudioHalAidlTest
Merged-In: I9c4a045bb5dba2b33acb65f7aba1a447a3dac4d0
Change-Id: I9c4a045bb5dba2b33acb65f7aba1a447a3dac4d0