MIDI: Add virtual devices per user

Create virtual MIDI devices per user.

Before, only the system user could create virtual MIDI devices.
These devices could be accessed by all users.
No other user could create virtual devices.

With this change, all users can create can install an app that provides
a Virtual MIDI Device Service.
getDevices() surfaces devices that can be opened by that user.
openDevice() rejects devices that are not created by that user.

Users can no longer open virtual MIDI devices created by other users.

Bug: 279553006
Bug: 272175328
Bug: 28673954
Test: atest CtsMidiTestCases
Test: Midi Synth Ex app  with Midi Keyboard app
Test: Installing virtual midi apps on multiple users
Change-Id: I7a2cf9a788cf1ccb701ef6be63295f336a479fe4