HAL out_get_parameters: do not return unknown keys

in_get_parameters and adv_get_parameters only return keys that are
supported. That allows the client the discriminate between keys not
supported and empty return value.

Align out_get_parameters with this behaviour.

Test: vts-tradefed run vts --module VtsHalAudioV2_0Target
Test: call/play music/record/video...
Bug: 36311550
Change-Id: Ia6e26dd50b05188099f4414d3c5a3c55f0ea89fa
Signed-off-by: Kevin Rocard <krocard@google.com>