Start Metalava integration for API doc generation

Currently this only works on Android projects (which are the overwhelming majority) and requires an explicit opt-in flag to be set in their build.gradle. Java library support and automatic use will follow.

Bug: 111261643
Test: ./gradlew :core-ktx:updateApi
Test: ./gradlew :core-ktx:checkApi
Change-Id: Ia0b8d9a8ac6fca64c3d28889771a49bc2e9e382b
diff --git a/buildSrc/src/main/kotlin/androidx/build/SupportLibraryExtension.kt b/buildSrc/src/main/kotlin/androidx/build/SupportLibraryExtension.kt
index 98528bb..f19900c 100644
--- a/buildSrc/src/main/kotlin/androidx/build/SupportLibraryExtension.kt
+++ b/buildSrc/src/main/kotlin/androidx/build/SupportLibraryExtension.kt
@@ -37,6 +37,8 @@
     var failOnUncheckedWarnings = true
     var failOnDeprecationWarnings = true
 
+    var useMetalava = false
+
     /**
      * This flag works only if publish flag is "true".
      * It is useful for modules that are used for tooling. For example room annotation
@@ -71,4 +73,4 @@
 class License {
     var name: String? = null
     var url: String? = null
-}
\ No newline at end of file
+}