Separate system_ext_service_contexts out of system sepolicy.

Bug: 137712473
Test: boot crosshatch
Change-Id: If483e7a99dc07f082dd0ecd0162a54140a3267de
diff --git a/Android.bp b/Android.bp
index e8bb270..b3e26ba 100644
--- a/Android.bp
+++ b/Android.bp
@@ -290,6 +290,12 @@
 }
 
 service_contexts {
+    name: "system_ext_service_contexts",
+    srcs: [":service_contexts_files"],
+    system_ext_specific: true,
+}
+
+service_contexts {
     name: "product_service_contexts",
     srcs: [":service_contexts_files"],
     product_specific: true,