Always pass --auto-add-overlay to aapt2 for aar prebuilts

Bug: 64723465
Test: Build all aar prebuilts in our internal tree
Change-Id: I82ef8d59d4f710b755226b2801dc65c2ccffe40c
diff --git a/core/prebuilt_internal.mk b/core/prebuilt_internal.mk
index 3622c9c..f515230 100644
--- a/core/prebuilt_internal.mk
+++ b/core/prebuilt_internal.mk
@@ -560,7 +560,7 @@
 
 # We needed only very few PRIVATE variables and aapt2.mk input variables. Reset the unnecessary ones.
 $(my_res_package): PRIVATE_AAPT2_CFLAGS :=
-$(my_res_package): PRIVATE_AAPT_FLAGS := --static-lib --no-static-lib-packages
+$(my_res_package): PRIVATE_AAPT_FLAGS := --static-lib --no-static-lib-packages --auto-add-overlay
 $(my_res_package): PRIVATE_ANDROID_MANIFEST := $(intermediates.COMMON)/aar/AndroidManifest.xml
 $(my_res_package): PRIVATE_AAPT_INCLUDES := $(framework_res_package_export)
 $(my_res_package): PRIVATE_SOURCE_INTERMEDIATES_DIR :=