RESTRICT AUTOMERGE: snapuserd: Run snapuserd threads in-process.

This removes the temporary daemon and socket and instead directly embeds
SnapshotHandlerManager. We can also remove the test flags for io_uring
as with this they're no longer necessary.

Bug: 269361087
Test: snapuserd_test
Change-Id: I728186d9bc90c98fabd76d3f86569840488ada96