Add getObjectSizeLong hidden API to MtpDevice class.

In the MTP spec, the object size is stored in MtpObjectInfo as unsigned
32-bit integer and fetched by the getObjectInfo operation. For the
objects that are more than 4GB, the object size is provided as one of
extra properties, which are fetched by different operation.

The CL adds to getObjectSizeLong hidden method to Java MtpDevice class
so that client code can obtain 4GB+ object size.

BUG=27805369

Change-Id: I8a5b85c8db39734f00e49709c61b271eb48ff33d