MtpStorage: correct the size of reserve space for MTP

According to description in frameworks/base/core/res/res/xml/storage_list.xml,
"mtpReserve: (integer) number of megabytes of storage MTP should reserve for free storage".
Mtpstorage class use it directly and doesn't change it to correct size in megabyes.
Mtp initor can not get correct storage information from android.

Change-Id: Icf59eb1eb478e67ea5990be96a9decb41aa55504
Signed-off-by: Bo Huang <bo.b.huang@intel.com>
Signed-off-by: Jack <Ren&lt;jack.ren@intel.com>
Signed-off-by: Bruce Beare <bruce.j.beare@intel.com>