Split MediaProvider into its own target.

As we move towards becoming a Mainline module, it's important that
we catch as many bugs as possible during initial code review, and
Error Prone is designed to do exactly that.

Instead of lumping ourselves into the generic "packages" grouping,
this change adds a specific package for this Mainline module.

Bug: 142561358
Test: ./build/soong/soong_ui.bash --make-mode MediaProviderLegacy com.google.android.mediaprovider MediaProviderTests RUN_ERROR_PRONE=true
Change-Id: I62102febe39225a54c77d9ad463c350b3df9cafa