Support including apk inside an apex

New property "apps" is added to APEX soong module. This property
configure a list of APKs to package inside APEX payload. For each
android_app module package it's APK into /app/$(android_app module Name)
directory.

Bug: 139906763
Test: m -j
Change-Id: Ic6655f211951af08097c76e157a07ebc52425718

java/app.go

@@ -139,6 +139,10 @@ func (a *AndroidApp) ExportedStaticPackages() android.Paths {
 	return nil
 }
 
+func (a *AndroidApp) OutputFile() android.Path {
+	return a.outputFile
+}
+
 var _ AndroidLibraryDependency = (*AndroidApp)(nil)
 
 type Certificate struct {