fix(app-bar): AppBarTitle now applies flex: 1 1 auto

This shouldn't really affect most users in any way since this only was
discovered with my documentation site's search behavior on mobile.

If you apply a `max-width` to an element and have `margin-left: auto`,
it won't always be displayed as `flex-end` which was the original
intention.

A side effect of this change is that you won't need to enable the
`first` prop on the first `AppBarAction` if there is an `AppBarTitle` in
your `AppBar`.

packages/app-bar/src/_mixins.scss

@@ -210,6 +210,7 @@
   @include rmd-utils-rtl-auto(margin-left, $rmd-app-bar-keyline);
   @include rmd-utils-hide-focus-outline;
 
+  flex: 1 1 auto;
   margin-bottom: 0;
   margin-top: 0;