PrintPreview: Fix "More options" a11y behavior to not read label twice.

Specifically:
 - Pass "More options" label as a <slot> to cr-expand-button.
 - Change md-select's aria-label to aria-labeledby

This results in better screen reader (VoiceOver) behavior where the
label is not read twice when navigating with Ctrl+Alt+RightArrow.

Bug: 1055829
Change-Id: I31dadd2e13ca8ce50e75050f0f580b06183bcfa0
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2510329
Commit-Queue: Rebekah Potter <rbpotter@chromium.org>
Auto-Submit: dpapad <dpapad@chromium.org>
Reviewed-by: Rebekah Potter <rbpotter@chromium.org>
Cr-Commit-Position: refs/heads/master@{#823422}

chrome/browser/resources/print_preview/ui/more_settings.html

@@ -17,7 +17,9 @@
   }
 
   :host cr-expand-button {
+    flex: 1;
     margin-inline-end: calc(var(--print-preview-sidebar-margin) + 6px);
+    margin-inline-start: var(--print-preview-sidebar-margin);
     --cr-expand-button-size: 28px;
   }
 
@@ -25,20 +27,13 @@
     display: none;
   }
 
-  #label {
-    color: var(--cr-primary-text-color);
-    flex: 1;
-    margin-inline-start: var(--print-preview-sidebar-margin);
-    text-align: start;
-  }
-
   :host([disabled]) #label {
     opacity: var(--cr-disabled-opacity);
   }
 </style>
 <div on-click="toggleExpandButton_" actionable>
-  <span id="label">$i18n{moreOptionsLabel}</span>
   <cr-expand-button alt="$i18n{moreOptionsLabel}"
       expanded="{{settingsExpandedByUser}}" disabled="[[disabled]]">
+    <div id="label">$i18n{moreOptionsLabel}</div>
   </cr-expand-button>
 </div>