MediaProvider: MediaScanner behavior on boot (2/3)

Summary:
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

Test Plan: Ensure it works

Reviewers: Mustaavalkosta, michyprima, rlasalle15

Differential Revision: http://review.codefirex.com/D142