Move GetDefaultDevice from AudioManagerMac to core_audio_mac.

GetDefaultDevice was a function local in AudioManagerMac. By moving it
to core_audio_mac everyone can get the default devices without having
access to an AudioManagerMac object.

I have moved GetDefaultDevice from audio_manager_mac.cc to
core_audio_util_mac.(h|cc). I've changed the function to return a
optional AudioDeviceID rather then the old function that took a pointer
and returned a boolean to tell if the pointer was updated. I've also
removed the thread check, since core_audio_util_mac does not have a
thread checker. Otherwise the new GetDefaultDevice is unchanged.

I've updated GetDefaultInputDevice()/GetDefaultOutputDevice() and in
AudioManagerMac to use the new GetDefaultDevice(). The thread check and
CHECK that was in the old GetDefaultDevice() are added into the
GetDefaultInputDevice()/GetDefaultOutputDevice() functions.

AudioManagerMac::GetDefaultDeviceID() is also updated to use the new
GetDefaultDevice(), but also changed to use
core_audio_mac::GetDeviceUniqueID(), rather then duplicating the code.

Bug: 442993607
Change-Id: I0610a1232560bb953dbc0c649f5c1bad45897390
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/7000248
Reviewed-by: Johannes Kron <kron@chromium.org>
Reviewed-by: Olga Sharonova <olka@chromium.org>
Commit-Queue: Olov Brändström <brandstrom@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1522972}