Replace Objects.firstNonNull() with MoreObjects.firstNonNull().

Objects.firstNonNull was deprecated in Guava 18 [1] and removed
in Guava 21 [2].

[1] https://github.com/google/guava/wiki/Release18
[2] https://github.com/google/guava/wiki/Release21

Change-Id: I8287d1766b6e30721cf732de661b6c659369ea4f