MediaSession2: Fix timing issue

Session/Controller needs mProvider. However, if the createProvider()
interacts with other components, than other components may use session
/controller object before mProvider is set.

This CL prevents such issues by calling initialize() to communicate
with other components after the provider is set.

Test: Run all MediaComponents test once
Change-Id: Ic6eb1a7f96a2084b3a011da30a5053aff5620977