[camera_android_camerax] Ensure startVideoCapturing and stopVideoRecording return video with expected duration

[camsim99]

flutter/packages#6697 makes an attempt to listen to CameraX VideoRecordEvents to ensure that startVideoCapturing and stopVideoRecording only return when the video recording process has actually started/stopped the video recording process, respectively. However, when testing this by recording a video for 2s, we found consistently that the duration of the video is shorter by a couple hundred of ms (1500/1400ms vs 2000ms), indicating that the process of starting/stopping video recording eats into the expected recording time.

We should identify whether the cause is within the CameraX library, the delay between Java/Dart communication, etc. and find a fix for this issue.

Note: Via testing, we found that camera_android has a similar delay, but it is slightly less (1800ms-ish vs 2000ms).