Add back fallback to the entry assembly for application name

[JunTaoLuo]

Original issue: #1137

[davidfowl]

We should fallback to Assembly.GetEntryAssembly().FullName

[JunTaoLuo]

I dont' think that's what we want:
Assembly.GetEntryAssembly().FullName gives testhost, Version=15.0.0.0, Culture=neutral, PublicKeyToken=b03f5f7f11d50a3a
I think we want:
Assembly.GetEntryAssembly().GetName().Name gives testhost

Note testhost is the assembly name as I'm debugging a test.

[JunTaoLuo]

Seems GetEntryAssembly() returns null when run inside a net461 test launched by xunit. It works fine for netcoreapp tests or executing samples targeting net461.

[Eilon]

Not planning to take this in the patch. The provided workaround in the original bug seems to be sufficient. If we see more people hitting this we will reconsider.