MIDI: Add test for MidiEventMultiScheduler

This CL adds a new folder in frameworks/base for MIDI tests.
This folder can act as a folder for unit tests.

A new test is added for MidiEventMultiScheduler.
com.android.internal.midi is used so this test has to be here.

Bug: 266473128
Test: atest MidiTests --rebuild-module-info
Change-Id: If0d33bd46f8c8adf3a3425888f469f942594aca9