Fix Unity 6 Android compatibility with gradle

Newer versions of Gradle require 'namespace' to be defined in the build.gradle.
Such as the one that ships with Unity 6.

Confirmed this change works with Unity 6.0.42f1 and is still compatible with the Unity's oldest LTS, 2021.3.45f1.

Author: jkasten2

com.onesignal.unity.android/Editor/OneSignalConfig.androidlib/build.gradle

@@ -1,6 +1,8 @@
 apply plugin: 'com.android.library'
 
 android {
+    namespace 'com.onesignal.onesignalsdk'
+
     sourceSets {
         main {
             manifest.srcFile 'AndroidManifest.xml'
@@ -21,4 +23,4 @@ android {
     lintOptions {
         abortOnError false
     }
-}
+}