Don't wait on lock in getClassLoader if already cached.

Currently, we are blocking on a lock which is not necessary if we have already cached the value. Chrome is seeing real-world pain from this - we have a background task that sits for a while in constructSplit (spinning up a large ClassLoader), and our main thread is blocking on this mLock for absolutely no purpose.

Change-Id: I9ddadde23ffd9e04988c8f1933d2c8652bc5f222