Revert "Remove permission android.permission.WRITE_MEDIA_STORAGE"

We need this to allow MTP to access secondary external storage devices

This reverts commit 35a2ea2f.

Conflicts:

	data/etc/platform.xml

Change-Id: I9faf54526182a786a1c0ca1fd01a09baabca186d
Signed-off-by: Mike Lockwood <lockwood@android.com>