Separate system_ext_file_contexts out of system sepolicy.

Bug: 137712473
Test: boot crosshatch
Change-Id: I09f63771d08ad18fb41fca801dd587b086be58c7
diff --git a/Android.bp b/Android.bp
index e1b57a5..5dd8628 100644
--- a/Android.bp
+++ b/Android.bp
@@ -199,6 +199,13 @@
 }
 
 file_contexts {
+    name: "system_ext_file_contexts",
+    srcs: [":file_contexts_files"],
+    system_ext_specific: true,
+    recovery_available: true,
+}
+
+file_contexts {
     name: "product_file_contexts",
     srcs: [":file_contexts_files"],
     product_specific: true,