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>