refactor(firebase_dart_flutter): add namespace to build.gradle

Author: rbellens

packages/firebase_dart_flutter/android/build.gradle

@@ -37,6 +37,8 @@ rootProject.allprojects {
 apply plugin: 'com.android.library'
 
 android {
+    namespace 'be.appsup.firebase_dart_flutter'
+
     compileSdkVersion 29
 
     defaultConfig {