snapuserd: Fix race condition in HandleManager shutdown.

When HandlerManager shuts down, the monitor thread is left detached. The
monitor thread does not hold a shared_ptr reference to the
HandlerManager, so the pointer can be left dangling.

Fix this by not detaching the monitor merge thread.

This patch also changes the test harness to destroy
SnapshotHandlerManager on "shutdown", to avoid state leaking into the
next instance of snapuserd.

Bug: 288273605
Test: snapuserd_test
Change-Id: Iaaf96a37657c85cff4d2a8b15ccfde4aa03d3220