Allow sdk_version: "none" to be used in prebuilts/

Bug: 156354511
Test: m nothing
Change-Id: Iffce78cbce88a8341146bf19e78acf25fc69f2b0
diff --git a/android/neverallow.go b/android/neverallow.go
index cf09792..f9fc03c 100644
--- a/android/neverallow.go
+++ b/android/neverallow.go
@@ -141,6 +141,7 @@
 		"external/icu",
 		"external/okhttp",
 		"external/wycheproof",
+		"prebuilts",
 	}
 
 	// Core library constraints. The sdk_version: "none" can only be used in core library projects.