Rename android.provider.Mtp to android.provider.Ptp

We are only supporting the PTP subset of MTP in host mode and have
no plans for full MTP, so calling it Mtp was misleading.

Also moved ACTION_MTP_SESSION_END Intent name to android.provider.MediaStore
and added it to the public API.

Change-Id: Ie35d15864d5972e65a39b982aa51316ecca3725a
Signed-off-by: Mike Lockwood <lockwood@android.com>