SystemReader: Offer methods with java.time API

SystemReader: Offer methods with java.time API

Error prone explains: The Date API is full of major design flaws and
pitfalls and should be avoided at all costs. Prefer the java.time
APIs, specifically, java.time.Instant (for physical time) and
java.time.LocalDate[Time] (for civil time).

Add to SystemReader methods to get the time and timezone in the new
java.time classes (Instant/ZoneId) and mark as deprecated their old
counterparts.

The mapping of methods is:
* #getCurrentTime -> #now (returns Instant instead of int)
* #getTimezone -> #getTimeZoneAt (returns ZoneOffset intead of int)
* #getTimeZone -> #getTimeZoneId (return ZoneId instead of TimeZone)

Change-Id: Ic55b2f442a40046ff0ed24f61f566fc7416471be