keep google api key out of git (revoked old key)

android/app/build.gradle

@@ -47,6 +47,7 @@ android {
         targetSdkVersion 29 // same as compileSdkVersion
         versionCode flutterVersionCode.toInteger()
         versionName flutterVersionName
+        manifestPlaceholders = [googleApiKey:keystoreProperties['googleApiKey']]
     }
 
 //    compileOptions {

android/app/src/main/AndroidManifest.xml

@@ -87,7 +87,7 @@
         </provider>
         <meta-data
             android:name="com.google.android.geo.API_KEY"
-            android:value="AIzaSyDf-1dN6JivrQGKSmxAdxERLM2egOvzGWs" />
+            android:value="${googleApiKey}" />
         <!-- Don't delete the meta-data below.
              This is used by the Flutter tool to generate GeneratedPluginRegistrant.java -->
         <meta-data