snapuserd: Move Process ops out of Worker.

These are so small they can be inlined into MergeWorker. Sharing these
methods will be difficult after decoupling from dm-user, since
acquisition of buffers will change.

Bug: 288273605
Test: snapuserd_test
Change-Id: I1625d1a6e55bcb2041f73453ca15a01f98263e8a