libprocessgroup: Move fd caching code into FdCacheHelper

Refactor file descriptor caching code and move it into FdCacheHelper
because later on when we introduce fd caching for SetProcessProfiles
the children of CachedFdProfileAction become different enough that
sharing the same parent becomes a hindrance.

Bug: 215557553
Signed-off-by: Suren Baghdasaryan <surenb@google.com>
Change-Id: If3812a0090c81a29e25f0888b0511cfaf48edea3
Merged-In: If3812a0090c81a29e25f0888b0511cfaf48edea3