Change exportedDirs and exportedSystemDirs from []string to android.Paths

exportedDirs and exportedSystemDirs are now changed to android.Paths so
that we can later manipulate the paths via Rel(), etc.

Test: m

Change-Id: I6fb02ea4983bcebac351bc284f75b44885379e8f
diff --git a/cc/androidmk.go b/cc/androidmk.go
index 9a98b0e..cdd8e92 100644
--- a/cc/androidmk.go
+++ b/cc/androidmk.go
@@ -148,10 +148,10 @@
 func (library *libraryDecorator) androidMkWriteExportedFlags(w io.Writer) {
 	exportedFlags := library.exportedFlags()
 	for _, dir := range library.exportedDirs() {
-		exportedFlags = append(exportedFlags, "-I"+dir)
+		exportedFlags = append(exportedFlags, "-I"+dir.String())
 	}
 	for _, dir := range library.exportedSystemDirs() {
-		exportedFlags = append(exportedFlags, "-isystem "+dir)
+		exportedFlags = append(exportedFlags, "-isystem "+dir.String())
 	}
 	if len(exportedFlags) > 0 {
 		fmt.Fprintln(w, "LOCAL_EXPORT_CFLAGS :=", strings.Join(exportedFlags, " "))