Gradle plugin for compiling and checking Stable AIDL

Copies a significant amount of test fixtures from AGP internals with
modifications to pass ktlint.

Updates buildSrc to make the plugin available within AndroidX.

Bug: 253427301
Test: StableAidlPluginTest, StableAidlCheckApiTest, StableAidlCompileTest
Change-Id: I4182f7988e2e84ee1e7f0b9d815ac54e249abf7c
diff --git a/settings.gradle b/settings.gradle
index 7b1d197..f66863f 100644
--- a/settings.gradle
+++ b/settings.gradle
@@ -1009,6 +1009,14 @@
 
 /////////////////////////////
 //
+// Plugins
+//
+/////////////////////////////
+
+includeProject(":stableaidl:stableaidl-gradle-plugin", [BuildType.MAIN])
+
+/////////////////////////////
+//
 // Samples
 //
 /////////////////////////////