Add filegroup for extservices file context

Add a filegroup for extservices so that it can be shared between the main
extservices apex and the one used for testing.

Bug: 138589409
Test: Manually
Change-Id: I2cca8a583b2aa72c8c29a32dd839fe599300b40f
diff --git a/apex/Android.bp b/apex/Android.bp
index 4a860e1..d3acfdb 100644
--- a/apex/Android.bp
+++ b/apex/Android.bp
@@ -180,3 +180,10 @@
     "com.android.tethering-file_contexts",
   ],
 }
+
+filegroup {
+  name: "com.android.extservices-file_contexts",
+  srcs: [
+    "com.android.extservices-file_contexts",
+  ],
+}