Use DEFAULT_TIDY_HEADER_DIRS env variable.

This variable is a regular expression to be passed as
additional patterns to the -header-filter flag for clang-tidy.
For example, when make with WITH_TIDY=1 DEFAULT_TIDY_HEADER_DIRS="d1/d2|mydir/*"
for a project in external/xyz, clang-tidy will be called with additional flag
  -header-filter=\"(external/xyz|d1/d2|mydir/*)\"

Test: make with WITH_TIDY=1 DEFAULT_TIDY_HEADER_DIRS="d1/d2|mydir/*"
Bug: 32668284
Change-Id: I6051f4f80bc6dbab882bd81435ccbc772772ac63

cc/config/tidy.go

@@ -61,7 +61,11 @@ func init() {
 	// Give warnings to header files only in selected directories.
 	// Do not give warnings to external or vendor header files, which contain too
 	// many warnings.
-	pctx.StaticVariable("TidyDefaultHeaderDirs", strings.Join([]string{
+	pctx.VariableFunc("TidyDefaultHeaderDirs", func(ctx android.PackageVarContext) string {
+		if override := ctx.Config().Getenv("DEFAULT_TIDY_HEADER_DIRS"); override != "" {
+			return override
+		}
+		return strings.Join([]string{
 			"art/",
 			"bionic/",
 			"bootable/",
@@ -74,7 +78,8 @@ func init() {
 			"libcore/",
 			"libnativehelper/",
 			"system/",
-	}, "|"))
+		}, "|")
+	})
 }
 
 type PathBasedTidyCheck struct {