Put composer 2.3 in the right cpuset.

the composer HAL should be in the system-background cpuset, but hwc 2.3 is not
in any cpuset. This patch restricts it to system-background.

BUG: 132559090
Test: Build, flash and boot.
Change-Id: I1739a6c049e195f6cc79b81fcba7a6fcb55b546a
diff --git a/graphics/composer/2.3/default/android.hardware.graphics.composer@2.3-service.rc b/graphics/composer/2.3/default/android.hardware.graphics.composer@2.3-service.rc
index 08e32d8..81ce890 100644
--- a/graphics/composer/2.3/default/android.hardware.graphics.composer@2.3-service.rc
+++ b/graphics/composer/2.3/default/android.hardware.graphics.composer@2.3-service.rc
@@ -4,3 +4,4 @@
     group graphics drmrpc
     capabilities SYS_NICE
     onrestart restart surfaceflinger
+    writepid /dev/cpuset/system-background/tasks