Project reorganization (#5096)

* Rename AndroidConfig to Config

* Move base AndroidManifest.xml

* Make fallback icons into an Android module

* Add example of how to only load a single extension in Gradle project

buildSrc/src/main/kotlin/Config.kt

@@ -0,0 +1,6 @@
+object Config {
+    const val compileSdk = 29
+    const val minSdk = 16
+    const val targetSdk = 29
+    const val buildTools = "29.0.3"
+}