Add null check before starting delayed cpuset task

This prevents a NullPointerException in onBootPhase when
CpuMonitorService fails to initialize mHandler during onStart.
Specifically, this can happen on systems where Cpuset and Cpufreq
features are not implemented, but they should not prevent Android
from booting up.

Bug: b/367393410
Change-Id: I02be8208ec1129e99f98992cf912c2138a36540a