add signing to buildGradleApp

pkgs/development/mobile/androidenv/build-gradle-app.nix

@@ -78,6 +78,25 @@ stdenv.mkDerivation ({
                           };
 
   buildPhase = ''
+    ${stdenv.lib.optionalString release ''
+      # Provide key signing attributes
+      ( echo "RELEASE_STORE_FILE=${keyStore}"
+        echo "RELEASE_KEY_ALIAS=${keyAlias}"
+        echo "RELEASE_STORE_PASSWORD=${keyStorePassword}"
+        echo "RELEASE_KEY_PASSWORD=${keyAliasPassword}"
+      ) >> local.properties
+
+      cat >>build.gradle <<EOF
+      signingConfigs {
+        release {
+          storeFile file(RELEASE_STORE_FILE)
+          storePassword RELEASE_STORE_PASSWORD
+          keyAlias RELEASE_KEY_ALIAS
+          keyPassword RELEASE_KEY_PASSWORD
+        }
+      }
+      EOF
+    ''}
     buildDir=`pwd`
     cp -r $ANDROID_HOME $buildDir/local_sdk
     chmod -R 755 local_sdk