Don't announce bouncer titles for a11y

This matches existing functionality and does not allow
for selection of the bouncer messages.

Fixes: 319558927
Fixes: 335539232
Test: manual - use Talkback on pin/pattern bouncer screens
Flag: ACONFIG com.android.systemui.revamped_bouncer_messages NEXTFOOD
Change-Id: Ic50c99b712425163cf8899259327f6dbdcd205fa