Incorrect file names in apkcerts.txt for Make modules

This change fixes a regression that was caused by I56bc55e2231eb946617a9d6f97d55c9b61f3bff7
that file names for Make modules are ".apk" in apkcerts.txt.

This was because PACKAGES.$(LOCAL_MODULE).STEM was only set for the
modules from Soong. It is now set for Make modules too.

Bug: 155440232
Test: OVERRIDE_TARGET_FLATTEN_APEX=true m apkcerts-list
and check the generated apkcerts txt file to see that it doesn't have an
entry where file name is ".apk"

Change-Id: I6f14bbb5678255bef2c1f2397f59ede6c2b5df18

core/app_prebuilt_internal.mk

@@ -172,6 +172,12 @@ else
   $(built_module): PRIVATE_CERTIFICATE_LINEAGE := $(LOCAL_CERTIFICATE_LINEAGE)
 endif
 
+ifneq ($(LOCAL_MODULE_STEM),)
+  PACKAGES.$(LOCAL_MODULE).STEM := $(LOCAL_MODULE_STEM)
+else
+  PACKAGES.$(LOCAL_MODULE).STEM := $(LOCAL_MODULE)
+endif
+
 include $(BUILD_SYSTEM)/app_certificate_validate.mk
 
 # Set a actual_partition_tag (calculated in base_rules.mk) for the package.

core/package_internal.mk

@@ -692,6 +692,12 @@ endif # LOCAL_PACKAGE_SPLITS
 PACKAGES.$(LOCAL_PACKAGE_NAME).OVERRIDES := $(strip $(LOCAL_OVERRIDES_PACKAGES))
 PACKAGES.$(LOCAL_PACKAGE_NAME).RESOURCE_FILES := $(all_resources)
 
+ifneq ($(LOCAL_MODULE_STEM),)
+  PACKAGES.$(LOCAL_MODULE).STEM := $(LOCAL_MODULE_STEM)
+else
+  PACKAGES.$(LOCAL_MODULE).STEM := $(LOCAL_MODULE)
+endif
+
 PACKAGES := $(PACKAGES) $(LOCAL_PACKAGE_NAME)
 
 # Reset internal variables.