avfvideosrc: fix build with older SDK not supporting AVDeviceFormat

AVDeviceFormat and AVFrameRateRange are available in iOS since 7.0
so we need a more dynamic approach to support compilation with older
SDK's. We use a NSObject to avoid referencing those types and key-value
coding or preformSelector to access properties.
avfvideosrc: fix build in iOS with isEqualTo doesn't exists