Resolve main issues with image transition animation
This MR saves the conversation activity list position (only when the activity restarts or is recreated when killed) which hopefully addresses the spurious item overlapping issues.
It also fixes an issue where the animation sometimes wasn't played when clicking on an image in a message with several other images.
Finally, it resolves the broken return animation which was caused by a forced downscroll that the BriarRecyclerView performed when the toolbar was hidden. This might also help with the item overlapping issues.
First part of #1475