Support transcode via MTP

If the persist.sys.fuse.transcode_mtp flag is disabled, we open files
via MediaProvider ContentResolver, this allows the MediaProvider
explictly disable transcode for the MTP process.

We are trying to optimize for the current default transcode option - B
which is to transcode by default. For this case we keep the original
path.

Note that this optimization also implies that if the user changes the
default option to A (not transcode) then no transcoding will happen even
if the user has enabled transcoding for MTP in the USB preferences.

Test: Manual
Bug: 158466651
Change-Id: I6ee8c0e74952fb4ddb94f4f2f37d108b8245dc3f