[SDL 0223] Add Currently Playing Media Image to MediaServiceData

[theresalech]

Proposal: Add Currently Playing Media Image to MediaServiceData

This proposal adds an image for the currently playing media item to the media app service.

Review: smartdevicelink/sdl_evolution#706

[theresalech]

There is not an immediate need to add media images as images for other service types is not currently supported in Manticore. We will consider adding images for all service types as time allows.