Respect `required` property of apex modules

This CL fixes a bug that `required` property of apex modules is not
reflected to the generated Android.mk file.

Bug: N/A
Test: m nothing
Change-Id: I36ccf538882f083f9433bd3eb05b3a5127f3822b
diff --git a/apex/androidmk.go b/apex/androidmk.go
index 94b8116..80237fb 100644
--- a/apex/androidmk.go
+++ b/apex/androidmk.go
@@ -326,6 +326,9 @@
 	var required []string
 	var targetRequired []string
 	var hostRequired []string
+	required = append(required, a.RequiredModuleNames()...)
+	targetRequired = append(targetRequired, a.TargetRequiredModuleNames()...)
+	hostRequired = append(hostRequired, a.HostRequiredModuleNames()...)
 	installMapSet := make(map[string]bool) // set of dependency module:location mappings
 	for _, fi := range a.filesInfo {
 		required = append(required, fi.requiredModuleNames...)