[bt][data] add PacketType to Channel classes

Add a PacketType trait type to l2cap::Channel and rfcomm::Channel, and
update SocketChannelRelay and SocketFactory to use that trait. This
eliminates one of the type arguments for each of those templates
(SocketChannelRelay and SocketFactory).

Bug: NET-1933
Test: bt-host-unittests (on eve)

Change-Id: Ic2a5c0cad8ed167910106bcf64d16123ae9a0cd9
11 files changed