Add ability to mark messages unread

[matthijskooijman]

This is essentially https://github.com/vector-im/riot-web/issues/4406 but for android:

As in other messaging applications, it would be nice to mark a particular message in a room as unread, returning to the state before it was read. This is helpful for when you need a future reminder to do something, or you didn't have a chance to read everything up to the present, and would like to move the unread marker back.

See the linked issue for additional discussion.

[bmarty]

I Copy the labels from https://github.com/vector-im/riot-web/issues/4406