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
diff --git a/core/static_java_library.mk b/core/static_java_library.mk
index af1ebfb..6e922c8 100644
--- a/core/static_java_library.mk
+++ b/core/static_java_library.mk
@@ -114,6 +114,13 @@
 endif
 endif
 
+ifdef LOCAL_USE_AAPT2
+$(intermediates.COMMON)/export_proguard_flags: $(addprefix $(LOCAL_PATH)/,$(LOCAL_EXPORT_PROGUARD_FLAG_FILES))
+	@echo "Export proguard flags: $@"
+	rm -f $@
+	cat $+ >$@
+endif
+
 # add --non-constant-id to prevent inlining constants.
 # AAR needs text symbol file R.txt.
 ifdef LOCAL_USE_AAPT2