Provides JDK 21 and an Android SDK (platforms 36/36.1, build-tools
36.0.0) via `nix develop`. Points AGP at the SDK's aapt2 and Gradle at
the Nix JDK, since the binaries they would otherwise download do not
run on NixOS.
Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
