Add BUILD_BROKEN_PLUGIN_VALIDATION flag

Use is to specify the list of plugins are are not allowed.

Ignore-AOSP-First: Requires an internal only change to not break when submitted
Test: CI
(cherry picked from https://googleplex-android-review.googlesource.com/q/commit:f24be9dba9b53b0c1682929253a3eba9af7a4c31)
Merged-In: Ia2fe83d82ead618e480e99a82b1b825ed08d3eab
Change-Id: Ia2fe83d82ead618e480e99a82b1b825ed08d3eab

Changes.md

@@ -1,5 +1,18 @@
 # Build System Changes for Android.mk Writers
 
+## Perform validation of Soong plugins
+
+Each Soong plugin will require manual work to migrate to Bazel. In order to
+minimize the manual work outside of build/soong, we are restricting plugins to
+those that exist today and those in vendor or hardware directories.
+
+If you need to extend the build system via a plugin, please reach out to the
+build team via email android-building@googlegroups.com (external) for any
+questions, or see [go/soong](http://go/soong) (internal).
+
+To omit the validation, `BUILD_BROKEN_PLUGIN_VALIDATION` expects a list of
+plugins to omit from the validation.
+
 ## Python 2 to 3 migration
 
 The path set when running builds now makes the `python` executable point to python 3,
@@ -15,7 +28,6 @@ overridden by setting the `BUILD_BROKEN_USES_SOONG_PYTHON2_MODULES` product conf
 variable to `true`.
 
 Python 2 is slated for complete removal in V.
-
 ## Stop referencing sysprop_library directly from cc modules
 
 For the migration to Bazel, we are no longer mapping sysprop_library targets