Don't animate screen brightness if there is a pending off transition

When the DisplayPowerRequest policy is POLICY_OFF, a screen state
transition animation to DISPLAY_OFF is intiated. However, in
updatePowerState() the screen brightness setting, based on the current
display state, is set to normal brightness and an animation is
triggered. This causes a transient flash on some devices, which are
transitioning from a dimmed screen to screen off.

This change checks for a pending screen off transition before
triggering a screen brightness animation, to prevent the flashes.

Bug: 18136235
Change-Id: I37f9fb28b3ec8a4fdbb45920c40d25ebd50c220b