Add merging of apexkeys and apkcerts metadata

Bug: 131437873
Test: Built system-only and vendor builds for merge. Compared
resulting apkcertx and apexkeys text files to that of a monolithic
build,
Test: Created colliding entries in both apexkeys and apkcerts text
files and ensure the script exited with an appropriate error message.
Test: Created unit tests to cover both non-colliding and colliding
entries
Change-Id: I6e42ce682ffa9059344e8cd63ba3a720c1f93452

core/Makefile

@@ -695,7 +695,7 @@ $(APKCERTS_FILE):
 	@echo APK certs list: $@
 	@mkdir -p $(dir $@)
 	@rm -f $@
-	$(foreach p,$(PACKAGES),\
+	$(foreach p,$(sort $(PACKAGES)),\
 	  $(if $(PACKAGES.$(p).EXTERNAL_KEY),\
 	    $(call _apkcerts_write_line,$(p),"EXTERNAL","",$(PACKAGES.$(p).COMPRESSED),$@),\
 	    $(call _apkcerts_write_line,$(p),$(PACKAGES.$(p).CERTIFICATE),$(PACKAGES.$(p).PRIVATE_KEY),$(PACKAGES.$(p).COMPRESSED),$@)))