Refactor proto in preparation for java proto support

Test: m -j checkbuild
Change-Id: Idf00ea0bacb2777458f9af2c7eb47e1e1854eeba
diff --git a/cc/library.go b/cc/library.go
index 1537fd4..577521c 100644
--- a/cc/library.go
+++ b/cc/library.go
@@ -655,8 +655,8 @@
 	if library.Properties.Proto.Export_proto_headers {
 		if library.baseCompiler.hasSrcExt(".proto") {
 			flags := []string{
-				"-I" + protoSubDir(ctx).String(),
-				"-I" + protoDir(ctx).String(),
+				"-I" + android.ProtoSubDir(ctx).String(),
+				"-I" + android.ProtoDir(ctx).String(),
 			}
 			library.reexportFlags(flags)
 			library.reuseExportedFlags = append(library.reuseExportedFlags, flags...)