Enable AndroidX for Flutter release builds

Author: wmioch

.gitignore

@@ -135,6 +135,7 @@ screenshots/
 *.keystore
 local.properties
 gradle.properties
+!/flutter_game/android/gradle.properties
 
 # Local Android SDK downloaded for manual builds/tests
 android-sdk/

flutter_game/android/gradle.properties

@@ -0,0 +1,4 @@
+org.gradle.jvmargs=-Xmx4G -XX:MaxMetaspaceSize=512m -XX:+HeapDumpOnOutOfMemoryError -Dfile.encoding=UTF-8
+kotlin.daemon.jvmargs=-Xmx4G
+android.useAndroidX=true
+android.enableJetifier=true