Create filegroup for sdkext file contexts

So that they can be reused in the test apex.

Bug: 137191822
Test: m
Change-Id: Iea23fee906d7c4dc9267bd3220f9d41c86fbb5db
diff --git a/apex/Android.bp b/apex/Android.bp
index 2ae2905..66ca3ec 100644
--- a/apex/Android.bp
+++ b/apex/Android.bp
@@ -133,6 +133,13 @@
 }
 
 filegroup {
+  name: "com.android.sdkext-file_contexts",
+  srcs: [
+    "com.android.sdkext-file_contexts",
+  ],
+}
+
+filegroup {
   name: "com.android.telephony-file_contexts",
   srcs: [
     "com.android.telephony-file_contexts",