And allow extensions to add their own media handlers.
I'm not too happy with the introduction of another global,
but didn't like the alternatives either:
* Add some hook to MockMediaHandlerFactory that would allow
extensions to add their own stuff in.
* Use another hook (like ParserTestTables or ParserTestGlobals)
and then override the service with a new instance - seemed
too hacky
The good thing about this is that it lets us kill off a class.
I'm other to other suggestions in case I missed something.
Bug: T169258
Depends-On: I5875621c58597426ad5242bf3d07714555c439b5
Change-Id: I1c2e903fb235395a8de8e0f7bf65ce07739d2930