Refactoring CL to improve names and allow to inject a VAD into `VadLevelAnalyzer` (new name for `VadWithLevel`). The injectable VAD is needed to inject a mock VAD and write better unit tests as new features are going to be added to the class. Bug: webrtc:7494 Change-Id: Ic0cea1e86a19a82533bd40fa04c061be3c44f068 Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/185180 Commit-Queue: Alessio Bazzica <alessiob@webrtc.org> Reviewed-by: Minyue Li <minyue@webrtc.org> Cr-Commit-Position: refs/heads/master@{#32195}