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This sets a proper default ducking multiplier used in Chromium's MediaSession
(see Chromium change 942055 for reference).

Two cases:

1)
On AThings speakers running CMA backend, which does its own ducking, disable
ducking in the Chromium MediaSession by setting its ducking multiplier to 1.0.

2)
For speakers not running CMA backend and relying on the Chromium MediaSession
to duck, set an appropriate ducking multiplier (-30dB) that is inline with
Cast's ducking levels [-35..-20]dB.

Note that this is a simplification of Cast's ducking logic and more fine-tuning
of this is desirable.

Bug: internal b/69630398, internal b/73907369
Test: Ran on Android Things speakers w/ and w/o CMA backend, observed proper
      ducking.

Change-Id: I5fe5a7e46750b88a7d44af1b38ec3f094a7c387d
Reviewed-on: https://chromium-review.googlesource.com/979261
Commit-Queue: Chris Kuiper <ckuiper@chromium.org>
Reviewed-by: Luke Halliwell <halliwell@chromium.org>
Cr-Commit-Position: refs/heads/master@{#545709}