Make ManagedServices more thread-safe

Added @GuardedBy annotations to all collection fields (according to their existing usage) plus a few synchronized blocks that were missing. ErrorProne now reports zero "GuardedBy" issues on this class.

Also added a test for a specific case that was found in the wild.

Fixes: 216508294
Test: atest
Change-Id: Ie221e467fd6fc8b510bc9b22c15abc242653c4b3