MtpServer: Do not attempt to stop MTP if it has not been started

Fixes an ANR that occurred if we got a USB disconnected event
before the MTP service had started.

Bug: 4118033

Change-Id: I6cad4281a5911a9926cae923f34d3a6bf98346c5
Signed-off-by: Mike Lockwood <lockwood@android.com>
1 file changed