Deprecate "speak passwords" setting.

This will now be controlled by individual accessibility services.
We'll provide the password information to them, and they can
present or hide the information as it makes sense for their users.

Password information was anyway provided when a headset was
connected.

Bug: 28139568
Test: Manually verified that TalkBack now speaks passwords on the
lock screen and in text views. Since I'm removing functionality
that didn't have tests, it's tricky to have specific tests.
Change-Id: Ic3c724ccce5762ee9dcd9e7dcbd4eae6734dd05e