MTP: Create files/folders with group media_rw instead of sdcard_rw

Otherwise the sdcard daemon that implements the Fuse file system at
/mnt/sdcard won't have write access to the files.

BUG: 3254123

Change-Id: Ib0c306bbc31ec936fd6164f17e7cd1b5dfff21b8
Signed-off-by: Mike Lockwood <lockwood@android.com>

media/jni/android_media_MtpServer.cpp

@@ -102,7 +102,7 @@ public:
             return false;
         }
 
-        mServer = new MtpServer(mFd, mDatabase, AID_SDCARD_RW, 0664, 0775);
+        mServer = new MtpServer(mFd, mDatabase, AID_MEDIA_RW, 0664, 0775);
         mServer->addStorage(mStoragePath, mReserveSpace);
         sMutex.unlock();