AnalogClock: Allow subclasses to override the "now" instant.

Allows you to use the (deprecated) AnalogClock widget to
display times other than the current system clock time.

While we're at it, update the AnalogClock widget assets to
contemporary vector drawables.

Bug: 177962166
Test: atest AnalogClockTest
Change-Id: I69e9e89b8f42030e910086ee79bb5a447a00bb84