Fix ArrayTypeMismatchException on Android

[AArnott]

.NET arrays are covariant, so a string[] can masquerade as an object[]. But apparently object[].AsSpan will fail on such a masquerading array, presumably because a Span<object> would allow an object to be written with no further type checking. But ReadOnlySpan<object> is safe over a string[], presumably because it is read only so the type incompatibility is of no concern.

In mono, our ParameterInfo[] array is actually an array of a derived type, so it throws. By creating a ReadOnlySpan<ParameterInfo> instead of a Span<ParameterInfo>, we avoid the exception.

Fixes #860