bootanimation: allow preload the bootanimation

This patch implements one simple performance enhancement, which
might benefit devices. It is completely opt-in by setting
the following variable.

  * TARGET_BOOTANIMATION_PRELOAD: will preload the bootanimation
    zip to memory; this is handy on devices where the storage
    might be slow. By preloading it, we get quick access to the
    zip file, and the animation doesn't stutter.

Conflicts:
        cmds/bootanimation/BootAnimation.cpp

Signed-off-by: Andreas Blaesius <skate4life@gmx.de>
- picked for L, original commit only applyed partially: https://github.com/CyanogenMod/android_frameworks_base/commit/e6b54405aa70d7503a114d9c90ef7518abdd7133 >

Change-Id: Ie92d1b0aa90b43ba39c368a2a33657a60c3a64b5