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
fix styles

Change-Id: Iba1474de4a3b2c7430ca8eb7f5c9b12a0454211a