Remove test code that depends on extension

Remove test code that depends on extension

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