Disable flattening only for unbundled apps.

This allows the unbundled master-art branch to still use flattened
APEXes in its test builds.

Bug: 159109002
Test: art/build/apex/runtests.sh on master-art
Test: m nothing
Change-Id: I768c6b7f5afbc348cd7730e4ba96507678434778
diff --git a/apex/apex.go b/apex/apex.go
index 58cbb13..e21cc5b 100644
--- a/apex/apex.go
+++ b/apex/apex.go
@@ -1866,7 +1866,7 @@
 }
 
 func (a *apexBundle) GenerateAndroidBuildActions(ctx android.ModuleContext) {
-	buildFlattenedAsDefault := ctx.Config().FlattenApex() && !ctx.Config().UnbundledBuild()
+	buildFlattenedAsDefault := ctx.Config().FlattenApex() && !ctx.Config().UnbundledBuildApps()
 	switch a.properties.ApexType {
 	case imageApex:
 		if buildFlattenedAsDefault {