bootanimation: allow animation to specify background color

Some OEMs want to have a bootanimation (or parts of it) displayed on a
color other than black. They currently just use full-screen frames for
that, which wastes lots of memory and bandwidth. This change allows
each part of the animation to specify a background color that will be
applied outside of the frame images; if unspecified the background
will be black as it is now.

Bug: 16635599
Change-Id: Ibf008fc75c5aad891c86ba9e4ec0879b7a61b8bd