Specify namespace to be compatible with AGP v8

See https://stackoverflow.com/questions/76108428/how-do-i-fix-namespace-not-specified-error-in-android-studio

Author: louischan-oursky

android/build.gradle

@@ -26,6 +26,7 @@ apply plugin: 'kotlin-android'
 
 android {
     compileSdkVersion 31
+    namespace = "com.authgear.flutter"
 
     compileOptions {
         sourceCompatibility JavaVersion.VERSION_1_8