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