Support required property

This adds LOCAL_REQUIRED_MODULES to the generated Android.mk for any
dependencies in the "required" list of a module.

Change-Id: Ibcfe89dce7d247eb441af94e37388a59e71a75a9
diff --git a/android/androidmk.go b/android/androidmk.go
index 1e261a0..0afa1bd 100644
--- a/android/androidmk.go
+++ b/android/androidmk.go
@@ -200,6 +200,10 @@
 	fmt.Fprintln(w, "LOCAL_MULTILIB :=", amod.commonProperties.Compile_multilib)
 	fmt.Fprintln(w, "LOCAL_PREBUILT_MODULE_FILE :=", data.OutputFile.String())
 
+	if len(amod.commonProperties.Required) > 0 {
+		fmt.Fprintln(w, "LOCAL_REQUIRED_MODULES := "+strings.Join(amod.commonProperties.Required, " "))
+	}
+
 	archStr := amod.Arch().ArchType.String()
 	host := false
 	switch amod.Os().Class {