libsnapshot:snapuserd: Fix memory leak

When worker threads were created, snapuserd was converted to a
shared_pointer. Earlier, memory was forcefully released
by setting snapuserd to nullptr which worked as it
was a unique pointer. Now, every worker thread holds
a reference. Clear the vector once all the worker
threads are terminated.

Test: Apply OTA and verify memory is released after OTA is applied
Bug: 183652708
Signed-off-by: Akilesh Kailash <akailash@google.com>
Change-Id: I256d26d98b02ad599aff49b92192226546c59b17