Fix default zen config to be up to date according to enabled flags

We were doing it only when reading XML, thus on next boot it would be resolved (but it looks bad).

Fixes: 363229699
Test: atest ZenModeHelperTest + manual (adding a new user)
Flag: android.app.modes_ui
Change-Id: I4f8b24b2ada51f642f3ad69a045347a2c0a21570