Add Backup Client

Currently WIP, a backup solution using the system backup service (same as "adb backup") integrated into system settings.

This is my first work on AOSP, feel free to tell me if anything is bad.

Also, english isn't my mother tongue, so please correct me if any of the strings sound bad.

Implemented:
	- Backup button and list of backups with restore/delete buttons in Settings -> Apps
	- Maximmum backup history setting in Settings -> Security

Known issues:
	- Backups for uninstalled apps can't be restored yet (only through "adb restore" because there's no list of all backups yet)
	- Sometimes after creating a backup in InstalledAppDetails, the back button does not work. Does anyone have an idea why that is?
	- The backup screen is always shown during backups, and you have to confirm every backup manually

Todo:
	- choose location
	- backup metadata
	- multi backup
	- multi restore
	- list of all backups
	- schedule full backup
	- copy/move desktop backup password from dev settings?
	- change backup screen text ("remote computer")
	- backup sdcard option
	- checkbox in uninstall dialog to delete backups