Performance/jank improvements to AnalogClock

This addresses rounding of the seconds hand fraction to avoid
advancing by inconsistent amounts.

It also moves to a less janky 30FPS update rate.

This also optimizes seconds ticking to only schedule callbacks as needed
and avoids spurious updates to the content description.

Also, potential NPE fix and dead code removed.

Bug: 186099601
Test: Local testing with sample app
Change-Id: I771b4c3f1d18f7836cd42202810a1495d25a9db5