ios: set default video constraints to 720p
ios: set default video constraints to 720p

By default the front camera  will be opened in 480p otherwise. This
aligns with the behavior in Android after
https://github.com/oney/react-native-webrtc/commit/f89f43c1799c2b65b99aa5d7a0114e0acf8eaf8d

zxcpoiu/react-native-webrtc,zxcpoiu/react-native-webrtc,zxcpoiu/react-native-webrtc,zxcpoiu/react-native-webrtc,zxcpoiu/react-native-webrtc,zxcpoiu/react-native-webrtc