[mscorlib/Android] Support TimeZoneInfo.FindSystemTimeZoneById("Local")

Fixes: https://bugzilla.xamarin.com/show_bug.cgi?id=24741

Xamarin.Android didn't previously support
TimeZoneInfo.FindSystemTimeZoneById("Local") because of oversight (and
overly nested #if conditionals).

Compare `id` against "Local" *before* doing platform-specific
codepaths so that finding the "Local" TimeZone works as expected on
Xamarin.Android, and merge the `#if MONODROID` with the existing code.