This CL adds a new interface CameraVideoCapturer that extends VideoCapturer with a switchCamera() function. It also moves moves CameraEventsHandler, CameraStatistics, and CameraSwitchHandler from VideoCapturerAndroid to this new interface. The purpose is to prepare for a camera2 implementation that will use the same interfaces and helper class.
BUG=webrtc:5519
Review-Url: https://codereview.webrtc.org/1895483002
Cr-Commit-Position: refs/heads/master@{#12723}
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