Add half-resolution boot animations for low-end devices

On low-end devices, the current 48 fps boot animation can use more than
50% of CPU time, and if the texture cache is enabled, a majority of main
memory as well.  For these devices, add half-resolution variants of the
lower-resolution boot animations which display 2x upscaled -- this
greatly speeds the boot process and makes the boot animation run more
smoothly.

Change-Id: I0140616ca38c52a06dd4622f1c20a9ca0da95f4b