ClatdController - populate mClatEgressMap

On egress we translate in the opposite direction
(from IPv4 to IPv6 instead of from IPv6 to IPv4), and thus key/value
must be reversed.

For now this map is the precise reverse of the mClatIngressMap,
but there will be further differentiation in the future.

Test: compiles, atest, flashed to phone on wifi/cellular ipv6 only
  network and observed dumpsys netd printout
Bug: 139396664
Signed-off-by: Maciej Żenczykowski <maze@google.com>
Change-Id: I65a8e85764aa24f2652f09b89273e4821ebc36fb