Bluetooth MIDI test: flush packets

The simulation of the device timing was not very accurate.
That caused two encoder tests to fail:
testTwoNoteOnsTwoChannels and testTwoNoteOnsSameChannel.

This patch simulates a delayed completion
and is more accurate.

Bug: 35669198
Test: atest BluetoothMidiTests

Change-Id: I8d04900a792ffc29928ed0e358536f8d961bcb77