Replace GATT_TRANSPORT=>BT_TRANSPORT

Add BT_TRANSPORT_AUTO for GATT_TRANSPORT_AUTO

Bug: 163134718
Tag: #refactor
Test: compile & verify basic functions working

g with '#' will be ignored, and an empty message aborts the commit.

Change-Id: I66683c6539dbac8c3f0dbc564c950a5315f8a7c0