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/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",