[2020-02] [mono][loader] Set status on success; avoid mmap on Android (#21610)

[2020-02] [mono][loader] Set status on success; avoid mmap on Android (#21610)

Manual backport of dotnet/runtime#80949 to mono/mono

Emebedders that call `mono_assembly_load_from_full` may observe a non-NULL return value and an uninitialized MonoImageOpenStatus, which may lead to incorrect diagnostics in code like:

```
MonoImageOpenStatus status;
MonoAssembly *assembly = mono_assembly_load_from_full (image, name, status, refonly);
if (!assembly || status != MONO_IMAGE_OK) {
   fprintf(stderr, "Failure due to: %s\n", mono_image_strerror (status));
   abort();
}
```
Which will print `Failure due to: Internal error`

Avoid an mmap that will fail since Android uses a custom loader and the assemblies aren't on disk

Co-authored-by: Aleksey Kliger <alklig@microsoft.com>