Export proguard flags from static android libraries and aar prebuilts

Test: m Contacts, classes-proguard.jar has same contents
Test: rm -rf out; tapas Contacts; m
Change-Id: Id7a92fb2c44913bccbfcbeae127355b4de588e4d
Merged-In: Id7a92fb2c44913bccbfcbeae127355b4de588e4d
(cherry picked from commit 1417c177ac9b942c4f1ae602f420444c72a90d6a)
diff --git a/core/clear_vars.mk b/core/clear_vars.mk
index 09f9be5..06518ee 100644
--- a/core/clear_vars.mk
+++ b/core/clear_vars.mk
@@ -78,6 +78,7 @@
 LOCAL_EXPORT_C_INCLUDE_DIRS:=
 LOCAL_EXPORT_HEADER_LIBRARY_HEADERS:=
 LOCAL_EXPORT_PACKAGE_RESOURCES:=
+LOCAL_EXPORT_PROGUARD_FLAG_FILES:=
 LOCAL_EXPORT_SHARED_LIBRARY_HEADERS:=
 LOCAL_EXPORT_STATIC_LIBRARY_HEADERS:=
 LOCAL_EXTRACT_APK:=