MediaProvider: MediaScanner behavior on boot [2/3]

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: Iad0b72f8b653b3d567b27de3ccf5c6cfb07e40ed
Add German translation for last commit

Change-Id: I588fd0fe283b509b6fd1e03c98d12bd58972a293