[fenix] For https://github.com/mozilla-mobile/fenix/issues/5104: Cleanup - Use match_constraint instead of match_parent.

Google officially recommends using match_constraint (equivalent to 0dp)
instead of match_parent when the view is contained in a ConstraintLayout.
https://developer.android.com/reference/android/support/constraint/ConstraintLayout#widgets-dimension-constraints

app/src/main/res/layout/fragment_site_permissions_exceptions.xml

@@ -12,7 +12,7 @@
 
     <androidx.recyclerview.widget.RecyclerView
         android:id="@+id/exceptions"
-        android:layout_width="match_parent"
+        android:layout_width="0dp"
         android:layout_height="wrap_content"
         android:layout_marginTop="16dp"
         android:visibility="gone"