MidiService: Add GuardedBy Annotation to USB MIDI

As a follow-up to ag/16835029, adding GuardedBy annotation
to new USB specific classes. This should help with identifying
potential races

Bug: 219763493
Test: Tested on MIDI 2.0 keyboard
Change-Id: I3d77db0d741cca8f79a759d2dcc99e8254b0298c
1 file changed