Separate system_ext_file_contexts out of system sepolicy.

Bug: 137712473
Test: boot crosshatch
Change-Id: I09f63771d08ad18fb41fca801dd587b086be58c7
diff --git a/build/soong/selinux_contexts.go b/build/soong/selinux_contexts.go
index 020357a..6259c9a 100644
--- a/build/soong/selinux_contexts.go
+++ b/build/soong/selinux_contexts.go
@@ -142,9 +142,10 @@
 			inputs = append(inputs, segroup.VendorSrcs()...)
 		} else if ctx.DeviceSpecific() {
 			inputs = append(inputs, segroup.OdmSrcs()...)
+		} else if ctx.SystemExtSpecific() {
+			inputs = append(inputs, segroup.SystemExtPrivateSrcs()...)
 		} else {
 			inputs = append(inputs, segroup.SystemPrivateSrcs()...)
-			inputs = append(inputs, segroup.SystemExtPrivateSrcs()...)
 
 			if ctx.Config().ProductCompatibleProperty() {
 				inputs = append(inputs, segroup.SystemPublicSrcs()...)