Fix thread safety issue on clearing cache

Historically, we havent held a lock on ForceClearCache and
ClearCacheIfNecessary. This is not thread safe because kAllPullAtomInfo
must be accessed in a lock, especially now that pullers can be
registered/unregistered.

Test: atest statsd_test, wrote a repro cl.
Bug: 168156854
Change-Id: I47d53a6d9d274bca4c78dbfd87e0097091b7b8cb