Grab audio focus when starting Voice actoins.

tested in pi (go/ag/3974655).

Bug: 76031221

Test: tested manually. Start Local media player. From
KitchenSink->HeadSet Audio->"Start Voice Interaction".
Ask a question. Local media should stop, Should hear the anwer.
and then Local media should resume playing.

Change-Id: Id80eeafafaf2204a637a1a298e5192787cb4cd12