Set activity title from preference screen title.

- When we initialize the preference screen, if the screen title is
available, use that to set the activity title, so that it will be up to
date with dynamic language changes.
- for preference fragment that don't have preference screen xml, added a
getTitle() method for the fragment to provide the title explicitly.
- use feature flag to switch between the old and new mechanism for
handling activity title.

Bug: 64564191
Test: blaze-bin/screenshots/android/i18nscreenshots/i18nscreenshots
Change-Id: I96318a5aa81e7b8ca94118adac3245ed27a03d93