Final push to build against SDK.

The bulk of the work needed to get MediaProvider building against
the "system_current" SDK surface has been slowly merged over the
last few months, and this change makes the last few adjustments.

This adds a new StorageVolumeCallback which is simpler version of
StorageEventListener that simply delivers the changed StorageVolume.

Move DownloadManager logic into a onMediaStoreDownloadsDeleted()
method which hides the implementation details of how the OS connects
with that implementation.

Make local copies of some ExifInterface parsing logic; they could
be added to the androidx version in an unbundled release.  Make a
local copy of RedactingFileDescriptor, since it's only needed for
the next few weeks until FUSE is globally enabled.

Bug: 137890034
Test: atest --test-mapping packages/providers/MediaProvider
Change-Id: Ib416eb8724781bdd234c8b7d728dee8b695ad6ac