Adding a system preference whether to speak passwords in accessibility mode.

By default we do not speak passwords if the user has no headset. However,
many users find this too restrictive and would like a way to enable
password announcement. While we cannot speak the passwords all the time
,to avoid leaking them, we expose a preference so each user can choose
the option that best works for him/her.

bug:5712607

Change-Id: I6eb0c40834abe5297f7dc74be02d180a5bef0174