Re-submit "Make mainline device use apex"

The build system default was changed to not support apex, but
we want the mainline device to enable it.

Test: make mainline_system
Change-Id: I9f29e8354acffb1856dfd8a173b80a3f9324630c
diff --git a/CleanSpec.mk b/CleanSpec.mk
index f43fa0e..b899bcd 100644
--- a/CleanSpec.mk
+++ b/CleanSpec.mk
@@ -646,6 +646,8 @@
 $(call add-clean-step, rm -rf $(PRODUCT_OUT)/odm/build.prop)
 $(call add-clean-step, rm -rf $(PRODUCT_OUT)/vendor/odm/build.prop)
 
+$(call add-clean-step, rm -rf $(PRODUCT_OUT)/system/apex)
+
 # ************************************************
 # NEWER CLEAN STEPS MUST BE AT THE END OF THE LIST
 # ************************************************