fix build in Omni 4.4

Change-Id: Ida3984a6b774c2da12342d48d8644cd18de7396c
diff --git a/Android.mk b/Android.mk
index dab1b17..1fc8c53 100644
--- a/Android.mk
+++ b/Android.mk
@@ -28,8 +28,10 @@
 
 ifeq ($(PROJECT_PATH_AGREES),true)
 
+ifeq ($(shell test $(PLATFORM_SDK_VERSION) -gt 20; echo $$?),0)
 # Make recovery domain permissive for TWRP
-BOARD_SEPOLICY_UNION += twrp.te
+    BOARD_SEPOLICY_UNION += twrp.te
+endif
 
 include $(CLEAR_VARS)