audio: Move metrics related code to audio_device_metrics_handler

No behavior change.

AudioDeviceMetricsHandlerTest

Bug: b/328425880
Test: AudioDeviceSelectionTest, CrasAudioHandlerTest,
Change-Id: I143ff7f6bfd780da1015041e0d1d29fb7597c5bc
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5420770


Commit-Queue: Wenyu Zhang <zhangwenyu@google.com>
Reviewed-by: David Padlipsky <dpad@google.com>
Cr-Commit-Position: refs/heads/main@{#1283106}