flutter: ci: Add release configs and secrets env

See:
https://dev.to/cddelta/signing-flutter-android-apps-for-release-in-github-actions-2892
https://flutterchina.club/android-release/

.github/workflows/flutter.yml

@@ -22,6 +22,9 @@ jobs:
       - uses: actions/setup-java@v1
         with:
           java-version: '12.x'
+      - run: echo $SIGNING_KEY | base64 -d > src/ui/flutter_app/android/app/key.jks
+        env:
+          SIGNING_KEY: ${{ secrets.SIGNING_KEY }}
       - uses: subosito/flutter-action@v1
         with:
           channel: 'dev' # or: 'dev' or 'beta'
@@ -51,6 +54,11 @@ jobs:
       # Build
       - name: Build apk
         run: cd src/ui/flutter_app; flutter build apk
+        env:
+          KEY_STORE_PASSWORD: ${{ secrets.KEY_STORE_PASSWORD }}
+          KEY_PASSWORD: ${{ secrets.KEY_PASSWORD }}
+          ALIAS: ${{ secrets.ALIAS }}
+          KEY_PATH: key.jks
 
       # Archive
       - name: Archive production artifacts

src/ui/flutter_app/android/app/build.gradle

@@ -24,6 +24,17 @@ if (flutterVersionName == null) {
 apply plugin: 'com.android.application'
 apply from: "$flutterRoot/packages/flutter_tools/gradle/flutter.gradle"
 
+def keystoreProperties = new Properties()
+def keystorePropertiesFile = rootProject.file('key.properties')
+if (keystorePropertiesFile.exists()) {
+    keystoreProperties.load(new FileInputStream(keystorePropertiesFile))
+} else {
+    keystoreProperties.setProperty('storePassword', System.getenv('KEY_STORE_PASSWORD'));
+    keystoreProperties.setProperty('keyPassword', System.getenv('KEY_PASSWORD'));
+    keystoreProperties.setProperty('keyAlias', System.getenv('ALIAS'));
+    keystoreProperties.setProperty('storeFile', System.getenv('KEY_PATH'));
+}
+
 android {
     compileSdkVersion 30
 
@@ -43,11 +54,17 @@ android {
         }
     }
 
+    signingConfigs {
+        release {
+            keyAlias keystoreProperties['keyAlias']
+            keyPassword keystoreProperties['keyPassword']
+            storeFile file(keystoreProperties['storeFile'])
+            storePassword keystoreProperties['storePassword']
+        }
+    }
     buildTypes {
         release {
-            // TODO: Add your own signing config for the release build.
-            // Signing with the debug keys for now, so `flutter run --release` works.
-            signingConfig signingConfigs.debug
+            signingConfig signingConfigs.release
         }
     }
     externalNativeBuild {