Add LOCAL_APKCERTS_FILE support.

This is used when Soong and Make don't know the full output file list,
and so use a tool to generate a per-module apkcerts.txt file instead.

Fixes: 160119159
Test: m apkcerts-list
Merged-In: I48183bc8cbf9dcc521f791812734205ed3f28f4c
Change-Id: I48183bc8cbf9dcc521f791812734205ed3f28f4c
diff --git a/core/clear_vars.mk b/core/clear_vars.mk
index c88a1cd..20319a8 100644
--- a/core/clear_vars.mk
+++ b/core/clear_vars.mk
@@ -22,6 +22,7 @@
 LOCAL_APIDIFF_OLDAPI:=
 LOCAL_APK_LIBRARIES:=
 LOCAL_APK_SET_MASTER_FILE:=
+LOCAL_APKCERTS_FILE:=
 LOCAL_ARM_MODE:=
 LOCAL_ASFLAGS:=
 LOCAL_ASSET_DIR:=