genrule: let Android.bp file specify exported header dirs

Instead of exporting the generated sources dir as headers, let
the Android.bp file specify subdirectories as exported.

Test: m -j checkbuild
Change-Id: I18dd900d63ce7485c8fbfcc39dc77abad6f733d7
diff --git a/cc/cc.go b/cc/cc.go
index 0071556..ddeede9 100644
--- a/cc/cc.go
+++ b/cc/cc.go
@@ -786,7 +786,7 @@
 				if genRule, ok := m.(genrule.SourceFileGenerator); ok {
 					depPaths.GeneratedHeaders = append(depPaths.GeneratedHeaders,
 						genRule.GeneratedSourceFiles()...)
-					flags := includeDirsToFlags(android.Paths{genRule.GeneratedHeaderDir()})
+					flags := includeDirsToFlags(genRule.GeneratedHeaderDirs())
 					depPaths.Flags = append(depPaths.Flags, flags)
 					if tag == genHeaderExportDepTag {
 						depPaths.ReexportedFlags = append(depPaths.ReexportedFlags, flags)