Add busytown scripts for androidx-metalava-integration

* Adds relevant busytown scripts to enable build
* Modifies the build to accept built-from-source metalava when
  applicable environment variables have been set

Test: Ran the scripts, they worked
Bug: 228469190

Change-Id: I8d5b60d0854ad3a0cb181891beab297d4305f4cf
diff --git a/settings.gradle b/settings.gradle
index a45a684..dc39e67 100644
--- a/settings.gradle
+++ b/settings.gradle
@@ -86,6 +86,11 @@
 dependencyResolutionManagement {
     versionCatalogs {
         libs {
+            def metalavaOverride = System.getenv("METALAVA_VERSION")
+            if (metalavaOverride != null) {
+                logger.warn("Using custom version ${metalavaOverride} of metalava due to METALAVA_VERSION being set.")
+                version('metalava', metalavaOverride)
+            }
             def skikoOverride = System.getenv("SKIKO_VERSION")
             if (skikoOverride != null) {
                 logger.warn("Using custom version ${skikoOverride} of SKIKO due to SKIKO_VERSION being set.")