This is just a "minor" upgrade for android "12L" - some tablet related
improvements we likely don't care about. This bump is to support a contribution
to migrate our UI from something legacy to an up-to-date default framework that
requires >=32.
- Use ndk 21 due to Go bug: https://github.com/golang/go/issues/42655
- Use pre-built clang binaries instead of building ourselves
- Build syncthing when creating container to prepopulate modules