Refactor LE disconnnect reason

When we initiate a disconnection, the reason should always be terminated
by local host.  We only need to check the reason when there is a
disconnection complete HCI event.  Therefore, we can get rid of
L2CA_GetDisconnectReason().

The only GATT app using disconnect reason is HOGP, and the only
interesting state is connection timeout.  Send this event from shim.

Test: cert/run
Tag: #gd-refactor
Bug: 141555841
Change-Id: I89973ece3ab5f24f3d1bc015daaf758d0600d002