The main advantage here is that we don't need any dependencies from
the unstable repo, which means we can be sure that the glibc version
we build against is suitable for Buster instead of being pulled in
from a later train. (We can't do this for Stretch because Stretch is
too old for all our build tools.)
While here, simplify the build a little bit: we're already using
snapshots of the Debian repo, so drop the separate file for pinned
dependencies.
- Switch to the modern maven-publish plugin.
- Bump the Android target SDK version to 30 to match the app.
(The minimum is still 19.)
- Bump the Java source compatibility version to 1.8.
- Bump the Android command line tools used in Docker to match the app.
- Bump the JDK used in Docker to OpenJDK 11, matching the app.
- Switch to the androidx testing libraries for emulator testing.
- Drop unused trove4j Gradle plugin.
- Lots of cleanup and refactoring.
Debian has a more stable retention period for pinned dependencies (the
version of OpenJDK 8 we were using for Ubuntu is gone already!), and
it matches what the Signal-Android repository is doing.
