Introduce ImGui Unity build

imgui/examples/example_android_opengl3/android/build.gradle

@@ -0,0 +1,24 @@
+buildscript {
+    ext.kotlin_version = '1.8.0'
+    repositories {
+        google()
+        mavenCentral()
+
+    }
+    dependencies {
+        classpath 'com.android.tools.build:gradle:7.4.1'
+        classpath "org.jetbrains.kotlin:kotlin-gradle-plugin:$kotlin_version"
+
+    }
+}
+
+allprojects {
+    repositories {
+        google()
+        mavenCentral()
+    }
+}
+
+task clean(type: Delete) {
+    delete rootProject.buildDir
+}