Add overlayable tag for NetworkStackConfig

This allows OEMs to customize the added resources with an overlay APK
installed in the product, system or vendor partition.

Bug: 111819230
Test: Installed test APK, verified resources changed. Resources not
      mentioned in overlayable file were not changed.

Change-Id: I24670005ea940d2ac29b8a0ff93e1559604fd3a8