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

- added xxhdpi icon
- no need for multiuser due that this should be a one time set
  general option set by the owner of the device

Change-Id: Iad0b72f8b653b3d567b27de3ccf5c6cfb07e40ed
MediaProvider: Allow scan for directories

Patchset 2: Allow only directories of external volumes

Change-Id: I5694f75896749d84b163a1213730eaa4c1e776e6
Signed-off-by: Jorge Ruesga <jorge@ruesga.com>
Revert "Allow holders of WRITE_MEDIA_STORAGE to write to secondary external storage"

This reverts commit b4171e508d37622774363d90bef16d76386eb44a.
MediaProvider: allow write access to external SD

Without this, Gallery cannot delete photos on external SD.

Change-Id: Ifbebcdb167cca92a5a05158fa4c8b81512b77ba6