MIDI: Fix UserManager.getMainUser() crash

There is no need to call getMainUser() when we can simply get the user
id from the app uid.

Bug: 293928587
Test: Connect Jamstick to MIDI+BTLE app
Test: Verify MidiScope sees the device and receives data
Change-Id: If2adb5118132dc445652c902ace29f0b1fccb057
(cherry picked from commit b6898d1c)