Avoid unnecessary FindClass("java/lang/String").

Use previously retrieved global reference instead.
Check for NULL after creating the global reference.

Change-Id: I21670d2ef0f63ddb5801dd21e7e56359e5519479
Fix possible integer overflow in MtpStorage

When converting the MB value MtpReserveSpace value to bytes there is a
possible integer overflow since 3 integer values are multiplied, even
though the result is stored in a long variable. This would be visible when
the value is more than 2 GB. This is solved by making the constants
longs instead of ints.

Change-Id: I799129f7076a6e331cece17b5c05aed210499648