[camera] Remove OCMock from AvailableCamerasTests, CameraOrientationTests and ThreadSafeEventChannelTests

[mchudy]

Follow-up on #8342. This should be merged after #8351

• Removes OCMock dependency in AvailableCamerasTests.m, CameraOrientationTests.m and ThreadSafeEventChannelTests.m
• Introduces new protocol wrappers to enable mocking FLTEventChannel and FLTCameraDeviceDiscovering

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[hellohuanlin]

Can we directly have AVCaptureDevice conform to FLTDefaultCaptureDeviceControlling protocol and save this boilerplate in this file?

[mchudy]

You're right, after playing around a bit I found that it is possible do that. I thought in ObjC that would be problematic and wanted to postpone it to the Swift migration (as Swift protocol extensions give much more flexibility). I had to change some other things like abstract AVCaptureInput creation to make this work, but it should be much cleaner now.