Disable the meaningless parts of the UI for A/B.

Bug: http://b/28748484
Test: mmma bootable/recovery
Change-Id: Ie86a265f4699503471e7c717d65677a916dbd25b
(cherry picked from commit 01fcbe160bd49891c0680b80c5688cd4feb2adbc)
diff --git a/Android.mk b/Android.mk
index 7fa98b0..15e281c 100644
--- a/Android.mk
+++ b/Android.mk
@@ -101,6 +101,10 @@
 LOCAL_C_INCLUDES += system/extras/ext4_utils
 LOCAL_STATIC_LIBRARIES += libext4_utils_static libz
 
+ifeq ($(AB_OTA_UPDATER),true)
+    LOCAL_CFLAGS += -DAB_OTA_UPDATER=1
+endif
+
 LOCAL_MODULE_PATH := $(TARGET_RECOVERY_ROOT_OUT)/sbin
 
 ifeq ($(TARGET_RECOVERY_UI_LIB),)