[Bluetooth] Remove preserve-focus attr from the BT device lists.

The preserve-focus attribute was clearing the menu item focus each time
the Bluetooth devices list updated. Removing this attribute fixes the
issue in a localized way. I tested explicitly to try to reproduce the
issue in crbug.com/699001 with the attribute removed. I did not see any
regression.

Fixed: 940062
Change-Id: I7e886959c7f4b4b604320b73f75a52d401d593da
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1894266Reviewed-by: Rebekah Potter <rbpotter@chromium.org>
Commit-Queue: James Vecore <vecore@google.com>
Cr-Commit-Position: refs/heads/master@{#711780}

chrome/browser/resources/settings/bluetooth_page/bluetooth_subpage.html

@@ -65,7 +65,7 @@
     <div id="pairedContainer" class="container"
         scrollable on-device-event="onDeviceEvent_"
         hidden="[[!showDevices_(bluetoothToggleState, pairedDeviceList_)]]">
-      <iron-list id="pairedDevices" preserve-focus items="[[pairedDeviceList_]]"
+      <iron-list id="pairedDevices" items="[[pairedDeviceList_]]"
           selection-enabled selected-item="{{selectedPairedItem_}}"
           scroll-target="pairedContainer" class="cr-separators">
         <template>
@@ -91,7 +91,7 @@
     <div id="unpairedContainer" class="container"
         scrollable on-device-event="onDeviceEvent_"
         hidden="[[!showDevices_(bluetoothToggleState, unpairedDeviceList_)]]">
-      <iron-list id="unpairedDevices" class="cr-separators" preserve-focus
+      <iron-list id="unpairedDevices" class="cr-separators"
           items="[[unpairedDeviceList_]]"
           selection-enabled selected-item="{{selectedUnpairedItem_}}"
           scroll-target="unpairedContainer">