Settings: 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: I266f0adc1aaf2b28a7735d5b9cc965e699f06a6c

Conflicts:
	res/values/arrays.xml
	res/values/strings.xml
	res/xml/development_prefs.xml
	src/com/android/settings/DevelopmentSettings.java