Make MR2ProviderServiceProxy take looper by param

The functionality doesn't change, because the calling function is
already running on the looper that the constructor gets. But this may
save someone from being bitten by the call to myLooper in the future.

Note that we avoid sharing the handler instance. Otherwise, one class
could inadvertently remove a message posted by the other class. And
sharing it would mean this change is no longer non-functional.

Bug: b/205124386
Test: Added logs to verify that myLooper was already the MediaRouterService looper thread.
Flag: EXEMPT refactor
Change-Id: Ia7f8dd472caa97162e61c28f56ef7e5fa8659309