MediaScanner behavior on boot (1/3)

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: I634ae96b1dcb493b966b4653589cd97523b63da7

Conflicts:
	core/java/android/provider/Settings.java

Conflicts:
	core/java/android/provider/Settings.java