[Media TTT] Add @SystemApis for all the media transfer callbacks.

This migrates the media transfer from just a service to a full-fledged
@SystemApi. Some of the testing infrastructure has not yet been migrated
to keep this CL slightly smaller.

Bug: 216318437
Test: systemui media.taptotransfer tests *still need to be updated*
Change-Id: Idf1f72de1343020668e158d74051c1dfb970cfcd