Fix confusing content settings names

[bbondy]

We currently use 'allow' for content setting of ad-block which is confusing because you're allowing the ad-block of ads.

Instead change to content settings name of 'ads' and allow or block them.

Also it's an issue if you toggle on and off in sequence for top level shields, it clears the saved previous values of the sub-shield settings. This needs to be stored separately.