[Network Details] Fix mobile data subpage when cellular and tether is on

This CL fixes issue with Mobile Data row in network settings opening
cellular network details page directly instead of the mobile data
subpage with cellular and tether network details. The issue was caused
because the network-summary component was incorrectly passing tether
device state to the row item.

Fixed: 1023674
Change-Id: Ib6cf971c82f79272a4e7c0d73a60603afa3285ad
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1916533Reviewed-by: Kyle Horimoto <khorimoto@chromium.org>
Commit-Queue: Azeem Arshad <azeemarshad@chromium.org>
Cr-Commit-Position: refs/heads/master@{#715746}

chrome/browser/resources/settings/internet_page/network_summary.html

@@ -12,7 +12,7 @@
             active-network-state="[[item]]"
             device-state="[[get(item.type, deviceStates)]]"
             network-state-list="[[get(item.type, networkStateLists_)]]"
-            tether-device-state="[[get('Tether', deviceStates)]]">
+            tether-device-state="[[getTetherDeviceState_(deviceStates)]]">
         </network-summary-item>
       </template>
     </div>