hal: Avoid multiple mixer open calls during init

audio_extn_utils_get_snd_card_num opens the mixer for
primary sound card. clients of this API are opening the
mixer again for the same card. Multiple mixer open calls
during HAL initialization are adding to boot up latency.

Return mixer handle as well along with the sound card
number from audio_extn_utils_open_snd_mixer. Added
close API as well to be consistent with the open API.
Retained old APIs for backward compatability with
ST HAL.

CRs-Fixed: 2205214
Change-Id: I323ee7a3c3fe446cd268b46a6e2c498467273ddb