audio: Introduce grace period when removing notification

The notification will be removed if the hotplugged device that triggered
the notification being disconnected. However, a USB device might be
plugged unfirmly, causing it disconnected and reconnected in a short
period of time. In this case, the notification should be remained.

Bug: b/328425880
Test: CrasAudioHandlerTest, test locally
Change-Id: I10004513e922467941accee15518ae3e79f25aa7
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5651362
Reviewed-by: Longbo Wei <longbowei@google.com>
Commit-Queue: Wenyu Zhang <zhangwenyu@google.com>
Cr-Commit-Position: refs/heads/main@{#1319309}