Add hwcomposer to the system-background cpuset for hal v2.2

This change was made for hal v2.1 but not cross-ported to 2.2.

Bug: 110167043
Test: $ adb shell ps -A | grep composer
system  630 ...  android.hardware.graphics.composer@2.2-service
$ adb shell cat /proc/630/cpuset
/system-background

Change-Id: I40b8a0386a7cf67fe64b1e0e40d81b78891faaca