fix(material/list): fix duplicate focus with chromevox on action items

For the list component, fix chromevox screenreader bug where it
duplicated focus on each item in the action list. After focusing on a
`button` in the action list, linearly navigating forward moved focus to
the `.mat-list-item-content` of the same item.

Fixes this by change the list item content from a `div` to `span`, since
we're generally not supposed to use div's inside buttons.

src/material/list/list-item.html

@@ -1,4 +1,4 @@
-<div class="mat-list-item-content">
+<span class="mat-list-item-content">
   <div class="mat-list-item-ripple" mat-ripple
        [matRippleTrigger]="_getHostElement()"
        [matRippleDisabled]="_isRippleDisabled()">
@@ -10,4 +10,4 @@
   <div class="mat-list-text"><ng-content select="[mat-line], [matLine]"></ng-content></div>
 
   <ng-content></ng-content>
-</div>
+</span>