Car emulator re-arrange volume groups and contexts

Moved system sounds contexts into alarms volume group, the main goals is
to separate the system sounds and the media sounds. Also moved call_ring
context into the same volume group with call context.

Bug: 186158497
Test: build and run car emulator.
Test: Play system sounds and make sure they have the same volume as
    alarms sounds.
Change-Id: I77049580d2d67d7f42342314fbf3977c58890e88