libfakeservicemanager: no hold lock in clear

destructors may reference servicemanager, and so
if objects are destroyed by libfakeservicemanager
clear, this was causing a recursive lock take.

Fix this the standard way, by using the lock to
copy out references, and then clear them when
its okay to talk to servicemanager again.

Bug: N/A
Test: w/ fuzzers
Change-Id: I4795ff6e042324e6ffe76f6c915c1328d3eee94f