Addressing ANR in BT-MIDI disconnect logic.

Also, don't attempt disconnect for non-BT-MIDI devices.

Bug: 216783547
Bug: 188924179
Bug: 216498842

Test: build, install, connect BLE-MIDI device. play a synthesizer to
verify

Change-Id: I7788264b304da2372da090c55db0a1e033c505d8