Video instruction step presentation improvements

[nicolelehrer]

• Tries to address //FIXME: video instruction step needs to adopt ORKInstructionStepContainerView so that video playback works

  • updates are made to stepContentView instead of instructionImageView, which is unavailable on stepView
  • adds a yOffset calculation so that play button image is centered vertically with thumbnail height

• Moves thumbnail calculation to background thread so video step presentation is not delayed

  • adds a placeholder image so that text doesn't bounce into place once thumbnail is created
  • once back on main thread, invokes stepDidChange again so presentation is updated

• Other updates

  • sets clipsToBounds to true, which seemed to be required for the image not to overflow after rotating to landscape
  • adds a scale to fill default content mode for the video instruction step's video thumbnail, instead of stretching to fill