Set LOCAL_APKCERTS_FILE for apk_set modules in apexes

I321e80fd636a955213761f56a3ac64bfe7f7f7c0 missed setting
LOCAL_APKCERTS_FILE for apk_set modules in apexes, which causes
invalid apkcerts.txt files.

Bug: 160652723
Test: m apkcerts-list
Change-Id: I1ccaafd04978374d0da10363a1d782cfeae428e7
diff --git a/apex/androidmk.go b/apex/androidmk.go
index 884a4e8..e299588 100644
--- a/apex/androidmk.go
+++ b/apex/androidmk.go
@@ -200,6 +200,7 @@
 				panic(fmt.Sprintf("Expected %s to be AndroidAppSet", fi.module))
 			}
 			fmt.Fprintln(w, "LOCAL_APK_SET_MASTER_FILE :=", as.MasterFile())
+			fmt.Fprintln(w, "LOCAL_APKCERTS_FILE :=", as.APKCertsFile().String())
 			fmt.Fprintln(w, "include $(BUILD_SYSTEM)/soong_android_app_set.mk")
 		case nativeSharedLib, nativeExecutable, nativeTest:
 			fmt.Fprintln(w, "LOCAL_MODULE_STEM :=", fi.Stem())
diff --git a/java/app.go b/java/app.go
index a0b208f..5af89b9 100755
--- a/java/app.go
+++ b/java/app.go
@@ -105,6 +105,10 @@
 	return as.masterFile
 }
 
+func (as *AndroidAppSet) APKCertsFile() android.Path {
+	return as.apkcertsFile
+}
+
 var TargetCpuAbi = map[string]string{
 	"arm":    "ARMEABI_V7A",
 	"arm64":  "ARM64_V8A",