floss: Refactor client for proper adapter mgmt

The adapter dbus proxy should only be created after the adapter is
enabled. This refactors the code to create the proxy object on the
OnHciEnabled callback.

As part of the refactor, a deadlock was discovered when trying to call
dbus apis from the callback. Thus, we added a new ForegroundActions
message loop so that we can trigger actions from callbacks.

Bug: 199213563
Tag: #floss
Test: Run btclient on chromeos
Change-Id: Ia7ca8af72cbed20474d8592d13e98b24dd6ce98b