Resolve issues with image attachment transitions

• Use a counter as the transition name in AttachmentItem.
• Tapping partly covered images makes them pop up from under the cover which looks glitchy
• Under mysterious circumstances messages can overlap each other (or leave gaps) after a return transition to the conversation
• On Moto G 4G (API 22), the exit transition still uses sliding
• Fix shared element exit transition when the list position is lost (activity was stopped or made into fullscreen)
• Change shared element of return transition when fullscreen view was swiped to another image when returning
• Return transition lands slightly below thumbnail when status bar is hidden

Subtask of #1237 (closed).