MidiService: protect private service info

Check for callers UID before returning MidiDeviceInfo.

Bug: 185796676
Test: see bug for repro steps
Test: atest CtsMidiTestCases
Test: https://source.android.com/devices/audio/midi_test.html
Change-Id: I4d5b8eca7457fa9b772e30e2f641bddb766177ee