Enforce LOCAL_CERTIFICATE is set in soong_app_prebuilt.mk

Missing LOCAL_CERTIFICATE produces an apkcerts.txt file that
has no key for the APK, which confuses the signing tools.
Enforce that it is set.

Bug: 147765187
Test: m apkcerts-list
Change-Id: I1299505d193deba5956954e5d9b6e4c727456b9c
diff --git a/core/soong_app_prebuilt.mk b/core/soong_app_prebuilt.mk
index ebb22c5..0a5ba9d 100644
--- a/core/soong_app_prebuilt.mk
+++ b/core/soong_app_prebuilt.mk
@@ -139,6 +139,9 @@
 my_2nd_arch_prefix :=
 
 PACKAGES := $(PACKAGES) $(LOCAL_MODULE)
+ifndef LOCAL_CERTIFICATE
+  $(call pretty-error,LOCAL_CERTIFICATE must be set for soong_app_prebuilt.mk)
+endif
 ifeq ($(LOCAL_CERTIFICATE),PRESIGNED)
   # The magic string "PRESIGNED" means this package is already checked
   # signed with its release key.