selinux: Only union sepolicy in SDKs 21,22

Manual union is not needed in Android 6.0+.

Change-Id: I7e321fb90b4333da349cc8ad8d2d78d990258b65
diff --git a/Android.mk b/Android.mk
index e0f7848..6e7478c 100644
--- a/Android.mk
+++ b/Android.mk
@@ -28,7 +28,7 @@
 
 ifeq ($(PROJECT_PATH_AGREES),true)
 
-ifeq ($(shell test $(PLATFORM_SDK_VERSION) -gt 20; echo $$?),0)
+ifneq (,$(filter $(PLATFORM_SDK_VERSION), 21 22))
 # Make recovery domain permissive for TWRP
     BOARD_SEPOLICY_UNION += twrp.te
 endif