Remove unnecessary locking from MediaScanner JNI code

There is nothing in this file that requires a global lock.
Furthermore, the lock can cause timeouts in native_finalize if one instance
is being garbage collected while another is busy in processDirectory

Bug: 5002520

Change-Id: I8717f8edeb932200d9f76b3d98915cfbe18b6ec6
Signed-off-by: Mike Lockwood <lockwood@android.com>