Replace hidden API startForegroundServiceAsUser

Replace it with getting context as the given user first and
calling startForegroundService, which is a public API.

Also add test for using PendingIntent#getService for
MediaButtonReceiver.

Bug: 161333794
Test: atest CtsMediaTestCases:android.media.cts.\
    MediaSessionTest#testSetMediaButtonReceiver_service
Change-Id: I0fbd2f5aaa011b1514ada6439816f89c4334f0a9