Don't try to be clever in onAmbientModeChanged().

http://developer.android.com/training/wearables/watch-faces/drawing.html
has code for onAmbientModeChanged() that tries to get the old ambient
mode by calling isInAmbientMode() before calling
super.onAmbientModeChanged().  This doesn't work, it already returns the
new ambient mode.  So the `inAmbientMode != wasInAmbientMode`
conditional is always false and all the ambient-mode-only code didn't
get executed.

Now, the appearance changes immediately after covering the watch with
your palm for a few seconds.
Try to fix time not updating in ambient mode.

The timer seems to not be called reliably in ambient mode.  Switch back
to onTimeTick() doing drawing in ambient mode, and disable the timer
while in ambient mode.

That's a bit of a bummer since the timer ticks less often than
onTimeTick(), but oh well.