bootanimation: add multithreaded decode

Some devices can't keep up on a single thread trying
to decode & display the frames at a high frame rate. This is
observed if the sleep delay between frames is negative:

01-02 04:29:25.114   530   542 D BootAnimation: 63, -22
01-02 04:29:25.176   530   542 D BootAnimation: 61, -20
01-02 04:29:25.248   530   542 D BootAnimation: 72, -30
01-02 04:29:25.315   530   542 D BootAnimation: 66, -24
01-02 04:29:25.381   530   542 D BootAnimation: 66, -24

To mitigate this, take advantage of multiple cores by decoding on
n cores and caching up to m images.  This keeps the memory footprint
small(ish) while still giving the best chance to maintain a
constant frame rate.

I measured boot time and fps for each animation part before
and after the change on an msm8909 with 1.5 GB RAM:

single thread:
01-02 04:40:45.826   540   556 I BootAnimation: fps = 22.40
01-02 04:40:49.457   540   556 I BootAnimation: fps = 13.22
01-02 04:40:51.464   540   556 I BootAnimation: fps = 23.92
01-02 04:41:19.375   540   556 I BootAnimation: fps = 22.89
01-02 04:41:23.942   540   556 I BootAnimation: fps = 15.55

  boot time: 51.05s

multi thread:
01-02 04:38:55.148   526   551 I BootAnimation: fps = 22.56
01-02 04:38:57.205   526   551 I BootAnimation: fps = 23.39
01-02 04:38:59.249   526   551 I BootAnimation: fps = 23.92
01-02 04:39:29.196   526   551 I BootAnimation: fps = 23.16
01-02 04:39:32.186   526   551 I BootAnimation: fps = 23.79

  boot time: 50.50s

Need to test the affect on boot time with an animation that doesn't
cache the textures as much as this to see the real effect.

Change-Id: If7464dc063b08a0bc33ee3f094028247b39650c1