Make prompt for headset while typing a password if accessibilbity is on less verbose.

If accessibility is enabled and there is no headset we do not speak the pressed keys.
In such a case we provide a prompt to the blind user to use a headset. This was announced
on every keypress which is quite annoying. Now this is announced only once.

bug:5342234

Change-Id: Ibe55ad991ad2153d09cde57b030544948fa0d73b