Remove -Xep:EqualsNull:WARN

This was temporarily demonted from an error
to a warning. Now that all the occurances
of this bug have been removed, make it an
error again.

Bug: 190944875
Test: Presubmits
Change-Id: Ic47d531f9dafcf34a14dc141dfc776e02bdd4484
1 file changed