MtpServer: Fix concurrent access to mStorages

 * getStorage is called both internally with mMutex held and externally
   without mMutex held.  Create getStorageLocked for internal use and
   make getStorage a wrapper.

 * hasStorage is only called internally with mMutex held.  Make it a
   private method.

Change-Id: I8f73310ad6cca14cd88b8e29f20cc181b3a4fac3