Cleanup NetlinkListener and NetlinkEvent

To make it easier to be compatible with older existing code, use
default parameters instead of separate functions for setting.

Also, reintroduce the const usage that was not in the original
changes by Stan.

Also fix some indent spacing.

Change-Id: Ice9ec7f0d5c4a0673037e2e04a764d88a98f68eb
Signed-off-by: Mike J. Chen <mjchen@google.com>