Make FakeAudioCaptureModule aware of the Init()/Terminate() methods.

This is needed so that the Initialize() method can be checked from tests
in between the Init() and Terminate() calls. Also checking for a call to
Init() to have been done before creating the internal test thread and
matching that by stopping processing and the thread from within
Terminate().

Bug: none
Change-Id: Iaa25821a74afb6acba919df21c8750e8735f2623
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/411620
Reviewed-by: Per Åhgren <peah@webrtc.org>
Auto-Submit: Tomas Gunnarsson <tommi@webrtc.org>
Commit-Queue: Tomas Gunnarsson <tommi@webrtc.org>
Cr-Commit-Position: refs/heads/main@{#45719}