Issue title: As a user, I want video to not autoplay when on mobile data
 Issue body: ### Why/User Benefit/User Problem
Mobile data is expensive, and even if the video is muted with today's current implementation, this still costs 💰 💰 .

### What / Requirements
Blocking auto-play video (not just muting it) when on a cellular connection.

### Acceptance Criteria (how do I know when I’m done?)
A preference in Settings that some combination of autoplay options: Allow, Only over Wi-Fi, Block, Never.

Notes:
- This is **NOT** the same as disabling autoplay video in a battery saving mode.
- Implementation detail: this should be possible with the site permission settings from GV/AC.

 Comments: 
 Comment 0: This is similar to #810
 Comment 1: @jonalmeida does this require any GV work to identify whether user is on mobile data or wifi?
 Comment 2: > @jonalmeida does this require any GV work to identify whether user is on mobile data or wifi?

@vesta0 Yes, this is a gecko feature.
 Comment 3: @cpeterso do we have a GV issue already to track this?
 Comment 4: > do we have a GV issue already to track this?

GV already has an API for this: setAutoplayDefault.

https://mozilla.github.io/geckoview/javadoc/mozilla-central/org/mozilla/geckoview/GeckoRuntimeSettings.html#setAutoplayDefault-int-
 Comment 5: > @cpeterso do we have a GV issue already to track this?

@vesta0 Oh heyyy, we already have this implemented! https://github.com/mozilla-mobile/android-components/blob/master/components/browser/engine-gecko-nightly/src/main/java/mozilla/components/browser/engine/gecko/GeckoEngine.kt#L183
 Comment 6: @cpeterso I was just a second too late to reply. :)
 Comment 7: Thank you @jonalmeida and @cpeterso ! So just to clarify, we already have an API to track when user is on mobile data?


 Comment 8: > we already have an API to track when user is on mobile data?

We have a GV API to toggle autoplay blocking. GV doesn't expose an API to track when the user is on mobile data, but Android does.
 Comment 9: @cpeterso thanks for the clarification.
 Comment 10: We need UX for this setting before we can work on it as the actual engineering change is pretty trivial.
 Comment 11: Will be covered by #255 
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