Android Pie unable to use front camera

recommended add camera settings,because mobile devices use Face (camera) need front camera.and the current code does not support Android Pie(9) using the front camera.because Android P use front camera requires `getUserMedia `to force use `facingMode: { exact: "user" }` or use `deviceId` to select,this is my solution for use deviceId in Android Pie(9)

```
tracking.initUserMedia_ = function(element, opt_options) {
window.navigator.mediaDevices.enumerateDevices().then(function(devices) {
let cfg={
video:{'facingMode':'user'},
audio: (opt_options && opt_options.audio) ? true : false,
};
devices = devices.filter(function(device) { return device.kind === 'videoinput'});
if (navigator.userAgent.toLowerCase().indexOf("android") > 0) {
for (let i = 0; i < devices.length; i++) {
let device = devices[i];
if (device.label) {
if (device.label.split(',')[1]==' facing front'){
cfg={
video:{
deviceId: {'exact':device.deviceId},
},
audio: (opt_options && opt_options.audio) ? true : false ,
};
break;
}
}
}
}
window.navigator.mediaDevices.getUserMedia(cfg).then(function(stream) {
element.srcObject = stream;
}).catch(function(err) {
throw Error('Cannot capture user camera.');
});
});
};
```