Fix some broken docs in android.media

- getInstance is SystemApi, so cannot be linked from public javadoc
- MidiDeviceService is not a member
- The documentation generation for @ChangeId constants is special
  and cannot resolve imports, so fully qualify all links there.

Bug: 303184203
Test: m docs
Change-Id: I5838dd3057e25d5c50f4b912d46c9805bb0378cf