Add support for LS Clock to use > 1 Paint object

Updated TextInterpolator and TextAnimator to support a
Paint object for each line of text. Now we can interpolate between
the colors when animating.

Clock colors are now based on the theme (for now, wallpaper text
color). ie: first line (of vertical clock) is one color, second line is
another color.

Also adjusted Keyguard clock logic to animate on doze changes instead of
animating incrementally on setDarkAmount.

Test: atest TextInterpolatorTest TextAnimatorTest
Bug: 170228350
Change-Id: Ia62f2de48534be2007afc5a95eac46b3c44ec880