Simplify handling of connection_complete

Reduce the possibility of accidentally failing to unblock the queue, by
splitting the callback into one function that handles connection
creation, and one that manages the state machine.

Tag: #refactor
Test: all existing
Bug: 246640776
BYPASS_LONG_LINES_REASON: Bluetooth

Change-Id: Id7aa897ede4ef9abeb26e60eedc1bb8e8953e2d2