fix(android): capture only stdout from nix build to fix SDK resolution on Linux

On Linux CI, Nix outputs warnings to stderr (e.g., "warning: Git tree is
dirty"). Using 2>&1 mixed these warnings into the captured path, causing
the directory check to fail and ANDROID_SDK_ROOT to stay empty. Changed
to 2>/dev/null so only the stdout store path is captured.

Also replaced android.sh build references in examples/android/devbox.json
with native gradle commands since android.sh build was removed.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

Author: abueide

examples/android/devbox.json

@@ -11,10 +11,10 @@
   "shell": {
     "scripts": {
       "build": [
-        "android.sh build"
+        "gradle assembleDebug"
       ],
       "build:release": [
-        "android.sh build --config Release"
+        "gradle assembleRelease"
       ],
       "start:app": [
         "android.sh run ${1:-${ANDROID_DEFAULT_DEVICE:-max}}"

plugins/android/virtenv/scripts/platform/core.sh

@@ -104,7 +104,7 @@ resolve_flake_sdk_root() {
   [ -n "${ANDROID_DEBUG_SETUP:-}" ] && echo "[CORE-$$] Building SDK: path:${root}#${output}" >&2
   sdk_out=$(
     nix --extra-experimental-features 'nix-command flakes' \
-      build "path:${root}#${output}" --no-link --print-out-paths 2>&1 || true
+      build "path:${root}#${output}" --no-link --print-out-paths 2>/dev/null || true
   )
   [ -n "${ANDROID_DEBUG_SETUP:-}" ] && echo "[CORE-$$] nix build returned: ${sdk_out:-(empty)}" >&2