Make bazel build //external/... work by only allowing "approved" existing BUILD files in the Bazel workspace.

Test: bazel build //bionic/... //external/... //frameworks/... //system/...

Test: ./build/bazel/scripts/run_presubmits.sh

Change-Id: I91865ca87c6535053e7a14d2526ff3ce0991bfea
diff --git a/bp2build/conversion.go b/bp2build/conversion.go
index eb83b38..101ad3d 100644
--- a/bp2build/conversion.go
+++ b/bp2build/conversion.go
@@ -59,7 +59,7 @@
 func createBuildFiles(buildToTargets map[string]BazelTargets, mode CodegenMode) []BazelFile {
 	files := make([]BazelFile, 0, len(buildToTargets))
 	for _, dir := range android.SortedStringKeys(buildToTargets) {
-		if mode == Bp2Build && !android.ShouldWriteBuildFileForDir(dir) {
+		if mode == Bp2Build && android.ShouldKeepExistingBuildFileForDir(dir) {
 			fmt.Printf("[bp2build] Not writing generated BUILD file for dir: '%s'\n", dir)
 			continue
 		}