Move media provider related hidden API flag file entries to packages/providers/MediaProvider

Bug: 179354495
Test: m com.android.mediaprovider
      - ensure that the generated APEX is byte-for-byte identical
        before and after these changes.
      m out/soong/hiddenapi/hiddenapi-flags.csv
      - make sure that they are not changed by this.
Change-Id: I0615b68715de2bb953da27b60d29d37b2750d86b