platform2: Replace 'OVERRIDE' with 'override' and fix linter

Replace OVERRIDE macro with native C++ 'override' keyword.
Also fixed the style issue of using both 'override' and 'virtual'
on member function overrides. Only one is required and cpplint
complains if both are specified now.

BUG=None
TEST=Build all affected target and ran unit tests which passed.

Change-Id: I04474aacad2ca8d6c6042c84a5378961d5bd8bf2
Reviewed-on: https://chromium-review.googlesource.com/212180


Reviewed-by: Alex Vakulenko <avakulenko@chromium.org>
Commit-Queue: Alex Vakulenko <avakulenko@chromium.org>
Tested-by: Alex Vakulenko <avakulenko@chromium.org>