Ensure mShowDialogs gets updated as part of global config update

The "App keeps crashing" dialog appears on Android TV even
though it should not. This would usually be accounted for
by setting mShowDialogs to false in the ActivityTaskManagerService
on boot. However, this does not happen because the service uses a
method of the ActivityTaskManager which pulls its configuration
from the context, which at this point is not yet updated to reflect
relevant values like the uiMode.

This change solves this problem by introducing an internal method
in the ActivityTaskManager, which acts on a given configuration
instead of the one from the context. This is helpful because the
caller ActivityTaskManagerService is holding on to the correct
configuration in the first place.

Furthermore, this change does not impact any outward-facing
behavior, nor does it introduce code duplication as the old
method of ActivityTaskManager new merely delegates its task
to the new one with the same configuration it would have
originally pulled from the context.

Test: Manually on ADT-3 device
Bug: 159019027
Change-Id: I0fac574a69a19243c2e62b967978ef5d8318ee51