bootanim: Add continuous splash support

 * We may or may not want to have a very smooth transition from the
   bootloader into Android by way of a continuous splash feature.
 * Add a compile-time flag which disables the initial screen clearing
   so that we don't drop a frame during the transition.

Change-Id: Ic453c901f1030ffebd9bdbeec59109bb5c585629
Merge remote-tracking branch 'cm/cm-11.0' into HEAD