fpc: keep fpc in system-background

The fingerprint HAL is insensitive to increased CPU throughput, but it also
has a tendency to spin while waiting for FP hardware. Limit FPC to the
system-background cpuset in order to avoid increased power consumption
when accidentally touching the fingerprint sensor.

bug 76115243
Test: fingerprint in system-background cpuset on taimen

Change-Id: Iaffe6f63bd76b7a1c4acaf0cae980840af515961