libprocessgroup: Add CgroupGetAttributePathForProcess and CgroupProcs attribute

ActivityManager monitors process forks with cgroup.procs in cgroup v2.
It currently generates its own paths to application cgroups, but v2
cgroup paths are owned by libprocessgroup. Those paths can change based
on build flags, so paths generated outside of libprocessgroup may not
always be correct.

Bug: 382693152
Bug: 381488882
Test: atest FrameworksMockingServicesTests:AppChildProcessTest
Test: atest CtsAppTestCases:ActivityManagerTest
Ignore-AOSP-first: Needed for framework changes which is run out of ag/
Change-Id: I005a114bf2610361098b7303a27dd7456eb469c8
Merged-In: I005a114bf2610361098b7303a27dd7456eb469c8