MediaScanner behavior on boot (3/3)

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: Ib09f2de2b3ae3e62eb59be681cf48e0bce14ef17

Conflicts:
	res/values/cr_arrays.xml
	res/values/slim_strings.xml
	res/xml/settings_headers.xml