Support Make rewriting APP-*

Test: ALLOW_MISSING_DEPENDENCIES=true m -j APP-Calculator
Change-Id: If5a0b60a0dbc615b6f47ea222616f8d34950ba0c
diff --git a/ui/build/make.go b/ui/build/make.go
index 2b39926..edf6d96 100644
--- a/ui/build/make.go
+++ b/ui/build/make.go
@@ -81,9 +81,10 @@
 	// Variables to export into the environment of Kati/Ninja
 	exportEnvVars := []string{
 		// So that we can use the correct TARGET_PRODUCT if it's been
-		// modified by PRODUCT-* arguments
+		// modified by PRODUCT-*/APP-* arguments
 		"TARGET_PRODUCT",
 		"TARGET_BUILD_VARIANT",
+		"TARGET_BUILD_APPS",
 
 		// compiler wrappers set up by make
 		"CC_WRAPPER",