Previously, we'd only show the attachment button when the user had
yet to enter any text. To add an attachment after text was entered,
you'd have to go to the three-dot menu. Now we just show a little
attach button in the text area.
I also took the opportunity to clean up other button paddings and
stuff in the compose area so things look better and react to text
sizes more predictably.
In particular, there were many issues with drawing corners.
Unfortunately, there's no pretty way to get masking working on every
Android version, so we have to switch back to using custom backgrounds
and then using multiple masking methods depending on Android version.
Also, I had to remove attr references in drawables. They crash on 4.x.
