Tilde Friends 0.0.21

This release was all about Android. After some non-trivial refactoring, I'm technically prepped to try to release in the app store, and I submitted a request to include in F-Droid.

• Reworked Android to launch its sandbox processes the prescribed way so that
  a Google Play-shippable android .aab is possible.
• Reworked the build to generate what F-Droid wants, too.
• Updated libbacktrace.
• Updated CodeMirror.
• Updated Android NDK.
• Enabled LTO for smaller code sizes.

Also there's some nominal CI: https://dev.tildefriends.net/cory/tildefriends/actions.

Updated!