Fixed a few issues with starting windows

1. startingData was accessed on a different thread without a lock held.
This caused synchronization issues where startingData could be set to
null while the starting window was getting created.

2. In onFirstWindowDrawn, startingData was not being set to null if the
startingWindow didn't exist. That means if the startingWindow hadn't
been created yet, it would get created after the first window was drawn.

Fixes: 129654434
Test: Steps from bug no longer reproducible
Change-Id: Ic5086798082b9f312cbd5069a937eac95cff7a9c