Remove unnecessary null checking on ViewRootImpl#mWindowAttribute

From static analysis, as ViewRootImpl#mWindowAttribute is now a final
object from CL[1] and the object is created when ViewRootImpl initialized
so the object reference should not be changed, it won't be necessary to do
the null check again for some code points.

[1]: I0d749c1abb38520fe8fc477d22d6523f470e9abc

Fix: 168335781
Fix: 168319140
Test: build and check launching app without any exception.

Change-Id: I30e1efb8e52704fbd8c97dc375e7ac253c6a90a8