Reload MediaBrowser children on each call to subscribe

Only one callback can be set on a given media id on MediaBrowser. When you
try to subscribe with a new callback it replaces the old callback, but it
wasn't resending the child data for the id. This causes calling subscribe
again also cause the data to be reloaded and sent to the app again.

bug:18683452
Change-Id: I2d30dbbbfe07cb9a720fb6d65664c1c4c58cb637