settings: Fix exception when search matches external storage management

Settings search relies on the HTML attribute associated-control to
show yellow bubbles when a search matches a sub-page.

Change the settings-subpage for external storage management to have
an associated-control at the top level of the settings page, so the
bubble appears on the Device > Storage row for a matching search.

This also prevents JS exceptions in the search code (the previous
selector didn't match any elements).

Test: On a device with Google Play Store support, open OS settings and
    search for "stor". Verify: JS console has no errors. Search field
    spinner eventually stops spinning. Yellow bubble appears over the
    Device > Storage item. Clicking search (X) clear bubble removes all
    yellow bubbles.

Bug: 1003970
Change-Id: I752eccebca599b1275fe505cff7cf561dc7ed66c
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1808179Reviewed-by: Michael Giuffrida <michaelpg@chromium.org>
Commit-Queue: James Cook <jamescook@chromium.org>
Cr-Commit-Position: refs/heads/master@{#697642}

chrome/browser/resources/settings/device_page/device_page.html

@@ -85,7 +85,7 @@
       <template is="dom-if" if="[[androidEnabled_]]">
         <template is="dom-if" route-path="/storage/externalStoragePreferences">
           <settings-subpage
-              associated-control="[[$$('#externalStoragePreferencesRow')]]"
+              associated-control="[[$$('#storageRow')]]"
               page-title="$i18n{storageExternal}">
             <settings-storage-external prefs="{{prefs}}">
             </settings-storage-external>