bootanim: Don't cache textures if they're expected to use a lot of VRAM

bootanimation cached all the textures generated until the animation
was completed.
For Hi-res animations with lots of frames (like the new CM9 anim on
xhdpi devices), this implied using large amounts of video RAM, more
than most devices actually have available, causing the animation to
stall (and in some cases, gralloc to lock up); so if an animation is
expected to use a lot of RAM (rough estimation based on frame size
and number of frames), disable cache entirely and generate the textures
on demand

Change-Id: I157e5cdde2aab1d82d980d77cbedd9f127c83a1d