audio_route: fix crash with unsupported control types

Check if control type is supported before accessing xxx_value
fields in mixer_state structure.

Also remove unused parameter warning.

Bug: 12871072.
Change-Id: I64c7d5e16728e3502f1fcd0a40658b72ec7acbe7
Merge remote-tracking branch 'aosp/kitkat-mr2-release' into blakandroid