ComputerVisionClient Error or Bug - ReadInStreamAsync method: 'Invalid URI format' in Xamarin.Forms

> Please provide us with the following information:
> ---------------------------------------------------------------

### This issue is for a: (mark with an `x`)
```
- [x ] bug report -> please search issues before submitting
- [ ] feature request
- [ ] documentation issue or request
- [ ] regression (a behavior that used to work and stopped in a new release)
```

### Minimal steps to reproduce
> I am capturing a photo and opening a stream using Xamarin.Essentials 1.7 MediaPicker built into Essentials.
When I call the ReadInStreamAsync(stream) method in Computer Vision Client, I get an error and my Xamarin.Forms app breaks inside the method: 'Invalid URI: The format of the URI could not be determined.'
This is the stream.Name value - '/data/user/0/com.companyname.xamphotoappdemo2/cache/2203693cc04e0be7f4f024d5f9499e13/198fd32db9cc4be38a493325974fa138/d964251252fc4963aca94339d73a8007.jpg'

var stream = await file.OpenReadAsync();

chosenImage.Source = ImageSource.FromStream(() => stream);

// 2. Add OCR logic.

var client = Authenticate(ApiSettings.subscriptionKey, ApiSettings.endpoint);

var text = await client.ReadInStreamAsync(stream);

//after the request, get the operation location
string operationLocation = text.OperationLocation;

//we only need the operation ID, not the whole URL
const int numberOfCharsInOperationId = 36;
string operationId = operationLocation.Substring(operationLocation.Length - numberOfCharsInOperationId);

//Get the ocr read results
ReadOperationResult results;
do
{
results = await client.GetReadResultAsync(Guid.Parse(operationId));
}
while ((results.Status == OperationStatusCodes.Running || results.Status == OperationStatusCodes.NotStarted));

var readResults = results.AnalyzeResult.ReadResults;

var expirationDates = from page in readResults
from line in page.Lines
where line.Text.Contains("EXPIRES") && line.Words.Count == 4
select line.Words[3].Text;

expirationDate.Text = expirationDates.ToString();

### Any log messages given by the failure
>

### Expected/desired behavior
> The text should be read in from the image and display on the Xaml page control.

### OS and Version?
> Windows 7, 8 or 10. Linux (which distribution). macOS (Yosemite? El Capitan? Sierra?)

### Versions
>

### Mention any other details that might be useful

> ---------------------------------------------------------------
> Thanks! We'll be in touch soon.

The issue names no repository file or test; start with the Xamarin.Forms call to ComputerVisionClient.ReadInStreamAsync and the reported stream.Name value. Reproduce the Invalid URI error with the provided MediaPicker stream, then verify that OCR reads the image and returns text without the exception.

由索引模型根据 Issue 内容生成。