libprocessgroup: limit libprocessgroup's VNDK API surface

Limit libprocessgroup VNDK API to the minimum set required for task
profiles usage. This API allows vendors to use cgroups without accessing
cgroup files directly, therefore allowing Android to change cgroup
arrangement details without breaking vendor code.

Bug: 131098932
Test: build and boot
Change-Id: I92463dfb44a108a133bafd2fe52237b6b1d50a69
Signed-off-by: Suren Baghdasaryan <surenb@google.com>