[System.Core/Android] Support Android v4.3's timezone DB format

Fixes: https://bugzilla.xamarin.com/show_bug.cgi?id=13686

Android v4.3 changed the timezone DB format described in 8a263984 and
replaced it with a single file containing version, index, data, and a
"zoneTab" (which is parsed but unused).

Port/rewrite the Java libcore.util.ZoneInfoDB type into
System.AndroidTzData so that this new file format is supported.

(Another day, another file format...)