Add preference to disable video autoplay

[cliambrown]

Relevant feature request: #1075

Untested: Waiting on Reddit to give me an API key (they seem to have made this process more complicated)

Similar to my previous PR #1323

• Adds pref_behaviour_video_autoplay (default = true to match current behaviour) with a corresponding checkbox on the "Images/Video" settings page
• Autoplay is only disabled if pref_behaviour_video_autoplay is false and playback controls are enabled (because I assume that there would be no way to initiate playback otherwise)
  • The checkbox is therefore disabled if pref_behaviour_video_playback_controls is false (btw, this could be applied to other relevant preference checkboxes using the android:dependency attribute)
  • I've added summary text to strings.xml explaining this limitation
• Autoplay is disabled in the ExoPlayerWrapperView constructor by calling mVideoPlayer.setPlayWhenReady(false) instead of (true) — can't be sure that this is the correct approach without an API key