floss: Check adapter enabled on init

btclient should check if the default adapter is already enabled on init
and initialize the dbus object if it is.

Bug: 199213563
Tag: #floss
Test: Enable adapter, close btclient, open btclient and verify adapter
      is still enabled.
Change-Id: I3c39022756b74f80577d7b8da2d824583b15c2bc