Optimize PrepareSnapshotPartitionsForUpdate runtime

During PrepareSnapshotPartitionsForUpdate, we attempt to connect to
snapuserd with a 5s timeout, only to tell snapuserd to shutdown
immediately. If snapuserd isn't running, we will wait-out the whole 5
seconds. Change the logic to return early if socket_connect() calls
return ENOENT, indicating that snapuserd socket isn't used by any
process. This reduces allocateSpaceForPayload() time from 6s to 1s.

Test: th
Bug: 315215541
Change-Id: Ib24d7c63733a896c082ac92aaa88ad52d050a2a5