MTP: Fix JNI warnings in MtpDevice

Needed to add a global ref to the classse to avoid
"not a valid JNI reference" errors when calling the constructor.

Change-Id: Ieb7e69f5d19fd5a090c53d32f69a201040c49112
Signed-off-by: Mike Lockwood <lockwood@android.com>
1 file changed