Rework thumbnail API to not suffer from IPC failures.

Thumbnails are now requested separately, so we don't exceed the
IPC buffer size limit.

Also implement issue #3349553a: Please provide a hook to intercept
fragment-breadcrumb clicks

And maybe fix issue #3439199: Music Notification does not turn on
when app switching out of Music app

Change-Id: Ie939e78cc8ded07b18112760e053185947549f61