Add hwcomposer to the system-background cpuset.

Little cores should be fast enough to handle hwcomposer work, so
avoiding using big cores for this is a potential opportunity to save
battery.

Bug: 73543056
Test: Verified using dumpsys gfxinfo that TouchLatency doesn't drop
frames.
Test: Took 10s systraces of TouchLatency and a Youtube VR video and saw
no frames dropped in SurfaceFlinger.

Change-Id: If96e13a2bacc9541f4d69a5736254817f20cacdd