Failure modes of `current_location` request not accomodated

### Describe the bug

According to the [Android documentation](https://developer.android.com/reference/android/location/LocationManager#getCurrentLocation(java.lang.String,%20android.location.LocationRequest,%20android.os.CancellationSignal,%20java.util.concurrent.Executor,%20java.util.function.Consumer%3Candroid.location.Location%3E)), it is possible for a one-time location request to fail; in which case, a location of `null` is returned. However, Toga's location service currently expects that the location is non-null.

### Steps to reproduce

This is difficult to reproduce specifically as it depends on a location lookup failure from the location service; however, if your code contains a one-time location request:
```
location = await self.location.current_location()
```

it is possible for the underlying `getCurrentLocation()` call to pass a value of `null` to the Consumer, resulting in the error:

```
E/AndroidRuntime: FATAL EXCEPTION: main
E/AndroidRuntime: Process: com.example.astrofilter, PID: 19840
E/AndroidRuntime: com.chaquo.python.PyException: AttributeError: 'NoneType' object has no attribute 'getLatitude'
E/AndroidRuntime: at .toga_android.hardware.location.toga_location(location.py:17)
E/AndroidRuntime: at .toga_android.hardware.location.accept(location.py:41)
E/AndroidRuntime: at .chaquopy_java.call(chaquopy_java.pyx:354)
```

### Expected behavior

If the location service can't obtain a GPS read, it should raise an error rather than crashing.

### Screenshots

_No response_

### Environment

- Operating System: Android
- Python version: All
- Software versions:
- Briefcase: 0.3.19
- Toga: 0.4.7+

### Logs

_No response_

### Additional context

iOS/macOS already handles this edge by raising a `RuntimeError` from the `locationManager:didFailWithError:` delegate method.

The Android testbed probe currently raises an xfail in the `simulate_location_error` handler; this should be returning a `None` result to the consumer (i.e., the same implementation as `simulate_current_location`, but passing a `None` value to the accept call).

Start with the Android location service in location.py and the testbed's simulate_current_location and simulate_location_error handlers. Compare the two probe handlers and trace how a null result reaches the consumer. Done means a failed request raises an error rather than crashing, with the testbed covering the null result.

Written by the indexing model from the issue text.