SF: make RenderEngineThreaded::waitUntilInitialized more efficient

RenderEngineThreaded::waitUntilInitialized is called from multiple
threads. Use atomic to avoid contending on a mutex.

Bug: 313924033
Test: presubmit
Change-Id: I4d61240a76f3ca814c28ccba62626f8b43a125cd
Merged-In: I4d61240a76f3ca814c28ccba62626f8b43a125cd
(cherry picked from commit 36135d0e)