[camera_android_camerax] Refactor camera state fields into camera state object #189599

Derived from flutter/packages#12145 (comment)

There are a bunch of variables in the camera_android_camerax plugin that track the state of the camera and its different use cases (image capture, video recording, etc.). To improve the clarity and readability of the state of the camera, we should refactor these into a CameraState sort of object that can respond to the different lifecycle events.