USB MIDI: Null check for mUsbMidiPacketConverter

Add null checks for mUsbMidiPacketConverter and
other variables inside input/output threads.

Also adding a NullPointerException catch for more
protection.

Also changing a timeout from 50ms to 200ms.

Bug: 233409535
Test: MidiScope and MidiKeyboard for MIDI 1.0/2.0
Change-Id: Ib73060d05ba8cbad9ab9b4d0e37a892d8916c0d9