Only listen to echo replies

This discards anything that is not an echo reply. Technically, length
cannot be 0, since zero-length packets close the PacketReader (because
FdEventsReader#shoudlProcessZeroLengthPacket has not been overridden
(yet?)).

Test: atest ApfIntegrationTest
Change-Id: Ib7cd9bf819614e66c8b6b7fc15622be5c344a208
1 file changed