L2cap: Auto send ConnectRsp

In the past, L2cap user can decide whether to accept or reject a
ConnectReq. If reject, the channel is closed.

Now let L2cap layer always auto accept the remote ConnectReq and start
config process. If the L2cap user wants to reject, they can send a
DisconnectReq. According to L2cap state machine, a DisconnectReq after
ConnectRsp terminates the channel, which is equivalent to sending a
negative ConnectRsp, as before.

Bug: 159815595
Tag: #refactor
Test: compile & verify basic functions working
Change-Id: Ibed56206148922356e8d9cb10650e9d82ab61225