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.
Unfortunately, the change wasn't as simple as just switching to use our
EmojiTextView. That view only supported single-line text. I added
support for multi-line text.
Fixes#7704.
