[mscorlib/Android] TimeZoneInfo.Local.Id should be "Local".

On Mono/.NET, TimeZoneInfo.Local has a TimeZoneInfo.Id value of
"Local" and a TimeZoneInfo.DisplayName value of "Local":

	$ csharp
	csharp> TimeZoneInfo.Local.Id;
	"Local"
	csharp> TimeZoneInfo.Local.DisplayName;
	"Local"

That isn't the case on Xamarin.Android, which returns the timezoneinfo
ID value from both DisplayName and Id, e.g. TimeZoneInfo.Local could
have Id and DisplayName values of "Australia/Sydney".

Rework things so that the TimeZoneInfo.Id and TimeZoneInfo.DisplayName
properties return "Local" on Xamarin.Android for the instance returned
from TimeZoneInfo.Local, just like normal Mono/.NET does.