Add Nix flake dev shell for building on NixOS

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>

flake.lock

@@ -0,0 +1,27 @@
+{
+  "nodes": {
+    "nixpkgs": {
+      "locked": {
+        "lastModified": 1791956154,
+        "narHash": "sha256-jBYGPi4udQ9uo4KQp9m2J0546nJVQitG15gvZSaVVAs=",
+        "owner": "NixOS",
+        "repo": "nixpkgs",
+        "rev": "55bfb4d7437bd4e09ab7cba58a43bd68eb310044",
+        "type": "github"
+      },
+      "original": {
+        "owner": "NixOS",
+        "ref": "nixos-unstable",
+        "repo": "nixpkgs",
+        "type": "github"
+      }
+    },
+    "root": {
+      "inputs": {
+        "nixpkgs": "nixpkgs"
+      }
+    }
+  },
+  "root": "root",
+  "version": 7
+}