Both Android and iOS using old .net 4.5 methods for
`Enum.GetValues` and `Marshal.GetFunctionPointerForDelegate`.
This commit updates them to use the modern
`Enum.GetValues<T>()` and `Marshal.GetFunctionPointerForDelegate<T>()`
methods.

We also update iOS to use `AppContext.BaseDirectory` instead of
`typeof(AL).Assembly.Location`, which is the correct property to
use in .NET 6+.