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
diff --git a/graphics/composer/2.1/default/android.hardware.graphics.composer@2.1-service.rc b/graphics/composer/2.1/default/android.hardware.graphics.composer@2.1-service.rc
index 3b6710b..5a5b51e 100644
--- a/graphics/composer/2.1/default/android.hardware.graphics.composer@2.1-service.rc
+++ b/graphics/composer/2.1/default/android.hardware.graphics.composer@2.1-service.rc
@@ -4,3 +4,4 @@
     group graphics drmrpc
     capabilities SYS_NICE
     onrestart restart surfaceflinger
+    writepid /dev/cpuset/system-background/tasks