Merge pull request #16964 from dominickuo/bug-973156-new-2

Bug 973156 - [Music] Send mozinterruptbegin and mozinterruptend to system for show/hide the media playback widget, r=jimporter(cherry picked from commit b83a79969718ea189cdc5cfa55d40fee38c11cb6)

Conflicts:

	apps/system/test/marionette/media_playback_test.js