Replaced deprecated "Time" with Calendar.

Some other minor tweaks:
- Clarified analog watch-hand trig, like changing
"minute / 30f * (float) Math.PI" to "minute / 60 * TWO_PI".
- Made the Canvas analog minute hands move on every redraw,
like a real analog watch.
- Replaced a deprecated getDrawable() call.
- Added 24-hour format support to digital clock.

Bug: 20012980
Change-Id: I58f070301c0d706b6c93794caa6fe3c310e605b1