Add getters to AudioPlaybackCaptureConfiguration

Unfortunately AudioMixingRule being in a different package,
some of its members had to be made public @hide to be used
by AudioPlaybackCaptureConfiguration.

Bug: 129948989
Test: atest AudioAttributesTest
Change-Id: Ie8cdf67aee9ff7cf9144a66d47c301712516cf87
Signed-off-by: Kevin Rocard <krocard@google.com>