android: Support images from keyboard

[gnprice]

This serves some of the same purposes as:

• Integrate finding a GIF to add to a message #418

but may be a lot easier to implement.

Android has built-in support for inserting images into a text box as a feature of the keyboard, and Flutter supports that since flutter/flutter#20796 . So we just need to wire that up for the message-content input box.

Chat thread: https://chat.zulip.org/#narrow/stream/137-feedback/topic/Mobile.20app.20GIF.20integration/near/1689275