MediaProvider: allow write access to external SD

Summary:
Without this, Gallery cannot delete photos on external SD.

Change-Id: Ifbebcdb167cca92a5a05158fa4c8b81512b77ba6

Test Plan: confirm and merge, thisbis ljely needed for all devices with external sd

Reviewers: synergy, randall.rushing, raymanfx

Reviewed By: synergy

Differential Revision: http://review.codefirex.com/D164