libcutils test: make set_sched_policy optional

set_sched_policy checks that operations related to the kernel config
CONFIG_CGROUP_SCHEDTUNE are working properly, but this config is not
required so devices without this were failing the test.

This CL splits set_sched_policy into two tests so that the policy and
timerslack tests can be skipped individually when devices don't support
them.

Bug: 113185515, 112550681
Test: libcutils_test{32,64} now pass on aosp_x86_64 emulator
Change-Id: I31638b61e033f0c96b63428a8d27d27dbc36bce3