Add API to call to vold for mounting OBBs

* Unhide StorageService class; hide all the USB-related items

* Add application-visible API to StorageManager for OBB files

* Add class for parceling OBB info across binders (ObbInfo)

* Add a JNI glue class to libutils/ObbFile (ObbScanner)

* Add API to MountService to deal with calling into vold and checking
  permissions

Change-Id: I33ecf9606b8ff535f3a2ada83931da6bbef41cfd