[generator] Strip arity from Cecil imported generic types.

[jpobst]

One of the issues trying to bind the BouncyCastle .jar in dotnet/android#11928 is that we write generic types with invalid signatures, like:

public List`1<string> MyProperty { get; set; }

This is how a Cecil type reference FullName is returned. Since this notation is not useful to us, this PR strips the arity while reading the assembly.

This results in the proper type syntax:

public List<string> MyProperty { get; set; }