DO NOT MERGE MTP and media provider support for multiple storage devices:

- MTP support for multiple storage units

- Add storage_id column to media database for MTP storage ID

- Add framework resource for defining mount points and user visible descriptions
for multiple volumes

- Clean up locking in MtpServer JNI code

Change-Id: I53d501fd4891ebe27408135fb598027e06b7e495
Signed-off-by: Mike Lockwood <lockwood@android.com>