Move keepalive constants to their right place

The constants CMD_{ADD,REMOVE}_KEEPALIVE_PACKET_FILTER are too
high in the file and not in order. These constants should be
moved back to their rightful place.

Bug: 123987395
Test: 1. m -j 2. m -j doc-comment-check-docs
Change-Id: I44c827d3a2011cf7c66c0444566e14192fec1b1b