debuggerd: Use libprocessgroup to unfreeze

Cgroup v2 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
Test: adb shell debuggerd -b <pid>
Change-Id: I7e486ab6f4068d0fae1be033a91b9a307f54ed42