fix(material/snack-bar): fix Firefox/JAWS not reading out snackbar message

Author: annieyw

src/material-experimental/mdc-snack-bar/snack-bar-container.html

@@ -10,6 +10,6 @@
     </div>
 
     <!-- Will receive the snack bar content from the non-live div, move will happen a short delay after opening -->
-    <div [attr.aria-live]="_live"></div>
+    <div [attr.aria-live]="_live" role="alert"></div>
   </div>
 </div>

src/material/snack-bar/snack-bar-container.html

@@ -4,4 +4,4 @@
 </div>
 
 <!-- Will receive the snack bar content from the non-live div, move will happen a short delay after opening -->
-<div [attr.aria-live]="_live"></div>
+<div [attr.aria-live]="_live" role="alert"></div>