GREF logging is broken in multi-process apps

[jonpryor]

Scenario: Create a Xamarin.Android app, add a e.g. Service, and set ServiceAttribute.Process:

[Service(Process="example")]
public class MyService : Android.App.Service {
    // ...
}

The problem is that the debug.mono.log system property is read at process startup -- for both (all) processes, the "normal" one and the "example" one, and they don't know to cooperate with each other.

The result is that they each write to grefs.txt, corrupting the contents.