More robust DRM support, including tests.

Based on guidance from the DRM team, we should pass DRM files
directly to MediaMetadataRetriever, which can be used to obtain both
the underlying MIME type and the typical metadata to be indexed.

To help ensure this works correctly and doesn't regress, borrow
strategy from CTS which creates new Forward Locked media items for
testing, and verify that we scan them correctly.

Allow MediaMetadataRetriever to override the MIME type obtained
from a file extension, but only when the primary type agrees with
the extension.  This is the same policy we apply to XMP metadata.

Bug: 141146658
Test: atest --test-mapping packages/providers/MediaProvider
Change-Id: I494a539a81ed39411a3b59c4433b6520967962c2