Move MediaStore.java inside APEX boundary.

Recent work has paved the way to get MediaStore.java building against
"core_platform", and this change is actually shifting MediaStore.java
inside the MediaProvider APEX boundary.

This involves defining a new "updatable-mediaprovider" library JAR
and ensuring that it's spliced into classpaths where needed to keep
everything building and working.

Note that the MediaProvider APK itself is still bundled, so we're
manually including the MediaStore.java when building that APK so that
we can continue referencing @hide symbols, but there's a STOPSHIP
comment to remove that once we get the APK building against
"system_current".

Bug: 144247087
Test: atest --test-mapping packages/providers/MediaProvider
Change-Id: I6d6da165848b934f4ca7ecfca7b97de6f97897e3