Use "override" keyword instead of "virtual".

"override" provides a compile-time gaurantee that the function is indeed
overriding a virtual function. This prevents the potential mistake of
creating a new virtual function rather than overriding the original.

I happened to notice we could use "override" here instead of "virtual".
Might as well tidy up a bit.

Test: Built locally
BUG=115613038

Change-Id: I7f43f4a466d8ceaa1b863d6a2af054e69618d0c8