Audio focus: stronger duck level for assistant use case

When audio focus is requested for usage ASSISTANT or
ASSISTANCE_NAVIGATION_GUIDANCE, with
AUDIOFOCUS_GAIN_TRANSIENT_MAY_DUCK, apply a stronger ducking
of 35dB (versus 14dB for "regular" ducking).

Bug: 256011854
Test: play driving directions during music playback
Change-Id: Ib9c6c0ba73911a810236a401419fb66798151b5f