Remove FLAG_SHOW_FOR_ALL_USERS to avoid crash in a11y

Text toasts now return null for getWindowParams() because they are
rendered by sys UI. Removing the flag for now to avoid crash. Working on
a long-term solution for this in the first bug linked.

Bug: 149408635
Bug: 149401510
Test: 1. Settings > Accessibility > Volume key shortcut > Use Volume key
         shortcut
      2. Press vol up and down together and observe no crash

Change-Id: Ie924666e457982986eb282d4cba7ba0beda6faca