Define policy for /proc/uid_procstat/set.

New procfs file written by the system_server to communicate fg/bg
state of UIDs to switch the statistics counter sets used.

avc: denied { write } for name="set" dev="proc" ino=4026531862 scontext=u:r:system_server:s0 tcontext=u:object_r:proc:s0 tclass=file permissive=1

Test: builds, boots, counter sets updated
Bug: 34360629
Change-Id: I2efbfbba9e73f50ce50a80a3dffd3b14fa55c048