[netemul] Fix sandbox service bindings teardown

- Recent refactor of sandbox_service.cc introduced a flake caused by the
destructor of SandboxBinding being called from another thread, the held
FIDL bindings in SandboxEnv were, consequently, being destroyed in the
wrong dispatcher.

BUG: FLK-195 #done

Offending test was failing once in ~30 runs. Ran test suite > 2000 times
to guarantee fix holds.

Change-Id: Ifa03c66ff8e8cd0991859e4fcd2b6463972d6f59