Preventing initialization exceptions in Xamarin client

Issue Description

Special configuration is required to prevent the following exception from being thrown during ParseClient initialization on Xamarin platform:

Value cannot be null. Parameter name: element

The issue is caused by an assumption encoded into the HostManifestData.Inferred property (Assembly.GetEntryAssembly() is non-null) that is not true on Xamarin. This has been noted in various issues such as #344.

Suggestions

Two suggestions come to mind to improve the situation:

1. Modify HostManifestData.Inferred so that it populates the Version, Name, and ShortVersion properties with some kind of reasonable default value (empty strings?) if Assembly.GetEntryAssembly() is missing instead of throwing an exception.
2. Modify HostManifestData.Inferred to throw an exception with a more meaningful error message to help Xamarin users realize that special configuration is needed.

Start with HostManifestData.Inferred and the ParseClient initialization path described in the issue. Review issue #344 for the existing Xamarin context and verify the behavior when Assembly.GetEntryAssembly() is unavailable. Done means choosing and implementing a defined fallback or a more useful exception without breaking normal initialization.