Optimise loading wallpaper from disk to memory using chunks.

Before this optimisation, target user's image wallpaper was being read
from disk to memory one byte at a time, which was causing too much
CPU consumption and slowness. With this CL, that process is optimised
by reading in 8 KB chunks via using the readAllBytes() function.

This CL decreases the average core (without the animations) user
switch duration by 32%, from 815.69ms to 556.66ms on a high end tablet
device. Impact is expected to be higher on lower end devices.

This CL also halves the memory consumtion since byte array is no more
getting duplicated.

Bug: 316331405
Test: Perfetto trace comparison
Flag: NONE
Change-Id: I1d6e556e51b0f81e18174f77edfc85ef7d8bb62a