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

services/core/java/com/android/server/media/MediaButtonReceiverHolder.java

@@ -221,8 +221,8 @@ final class MediaButtonReceiverHolder {
                         context.startActivityAsUser(mediaButtonIntent, userHandle);
                         break;
                     case COMPONENT_TYPE_SERVICE:
-                        context.startForegroundServiceAsUser(mediaButtonIntent,
-                                userHandle);
+                        context.createContextAsUser(userHandle, 0).startForegroundService(
+                                mediaButtonIntent);
                         break;
                     default:
                         // Legacy behavior for other cases.