bootanimation: Change bootanim ioprio to highest in IOPRIO_CLASS_RT

We see cases where bootanim is IO sleeping on boot and its causing
jank. Change the ioprio to the highest in an attempt to reduce this.

Bug: 138459662
Test: manual - flash and factory reset, looking for artifacts in the boot animation
Change-Id: I9734edea37c92e365a2b359a5180303e9b9284f7