Reapply "libprocessgroup: Remove __BEGIN_DECLS and __END_DECLS"

This reverts commit fdaaef95.

Reason for revert: should be safe to reapply post
  https://android-review.git.corp.google.com/c/platform/packages/modules/Connectivity/+/3369257



Test: TreeHugger
Bug: 379796721
Signed-off-by: Maciej Żenczykowski <maze@google.com>
Change-Id: I1a24791713484461345312a34777c2266e443cee