Fix NPE on rootViewImpl.getView

We have two instances which can (very rarely) trigger an NPE, when we
check if we have a non-null rootViewImpl with a non-null view, and use
that view later. This code is in a synchronized block, but that lock
does not protect the viewRootImpl's getView call, instead we should just
save a final reference to it.

Bug: 313399081
Test: n/a
Change-Id: I047b50d26972e6407b04d7cc09ef9110c5fd4b41