FloatingToolbar: Rules for ordering menu items.

- The assist menu item should always appear first
- Only the assist menu item should have an icon + text label

The description above enforces the expected UX without breaking
existing apps. Previously, apps couldn't have icons + text for a
menu item (we only rendered text for such items). If we don't
enforce that the assist menu item (android.R.id.textAssist) appears
first, then this item could potentially be rendered in a different
position for apps that specify an order number lower than that
specified for the assist menu item.

Test: Included in this cl.

Bug: 35989798

Change-Id: I56c71450c850e1d8cd80d54d4e17a6390823485f