Ensure classes are initialized rather than initializing.

A class can be being initialized on a different thread, in that case other
threads should block trying to access the class. The initializing state shows
the class is being initialized but not that its safe for other threads to
access. Change occurances of IsInitializing to IsInitialized primarily in
slow-path code.
Bug: 15347354

Change-Id: Ib586d0a385be6086a890dfbf8868d76f16767fac