Exported adapter object using --hci flag

When --hci=N is given in command line flags, dbus should export that hci
adapter's object (i.e. --hci=2 /org/chromium/bluetooth/adapter2). This
is necessary in order to support multiple adapters.

Bug: 193061807
Tag: #floss
Test: Verified on ChromeOS with dbus-monitor
Change-Id: I7986b349bfac18de44de57aa08ac2ab275f9b4bc