Opening the maps on R devices is unnecessary (because no code
uses them) and unsafe (because the maps might actually be in a
different format than expected by the module).
Always return null when opening the maps, so the code will safely
fall back to using netd for anything.
This only affects dump(). The production code never opens the
maps except via the shim, and only the S shim opens the maps.
Also check isAtLeastS in dumpCounters, because it opens the map
directly.
Bug: 177884581
Test: builds, boots, "dumpsys tethering" shows expected output
Change-Id: I5884490a5cc40fc529a12100ae5baaeae6a18f30
3c228f4923