- ContentProtection elements were missing when using MpdWriter,
therefore mpd_generator also did not generate ContentProtection.
- MpdWriter uses MpdNotifier now, instead of directly using MpdBuiler.
- Add mock_mpd_notifier.{h,cc} to be shared by several tests.
Change-Id: Id0f6213aa0c80e50294cd4193962054b0b5c2492