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: I98c64bcb538b6054299d1681055135fd1612577f
Revert "Disbale Lockscreen notification by default"

This reverts commit 46badc0994fc13030e5aeb00ce83a1d48aca4a5d.