MediaProvider: allow write access to external SD

Without this, Gallery cannot delete photos on external SD.

Change-Id: Ifbebcdb167cca92a5a05158fa4c8b81512b77ba6
MediaProvider: MediaScanner behavior on boot (2/3)

Lets you enable/disable/ask (with a notification) media scanning on boot.
Can speed up boot and reduce initial boot lag from MediaScanner

Change-Id: Iad0b72f8b653b3d567b27de3ccf5c6cfb07e40ed