Separate out Proguard flag files to fix dependency.

Bug: 3048005
Change-Id: I3e77ca1082cf7c92a43b249e5c887e3fbdf4ba11
diff --git a/core/clear_vars.mk b/core/clear_vars.mk
index 9681364..ee28f21 100644
--- a/core/clear_vars.mk
+++ b/core/clear_vars.mk
@@ -94,6 +94,7 @@
 LOCAL_NO_EMMA_COMPILE:=
 LOCAL_PROGUARD_ENABLED:= # '',optonly,full,custom
 LOCAL_PROGUARD_FLAGS:=
+LOCAL_PROGUARD_FLAG_FILES:=
 LOCAL_EMMA_COVERAGE_FILTER:=
 LOCAL_MANIFEST_FILE:=
 LOCAL_BUILD_HOST_DEX:=