floss: return the correct position for autocomplete

The function BtHelper::complete is supposed to return the position
of the current token, but currently it always returns 0.

Bug: 238572622
Tag: #floss
Test: Test these in btclient
      (1) a<Tab> => ad
                    adapter    advertise
      (2) adapter d<Tab> => adapter dis
                            disable    discoverable
      (3) adapter discov<Tab> => adapter discoverable

BYPASS_LONG_LINES_REASON: Bluetooth likes 120 lines

Change-Id: I22b551262656f0d5e8ad70cac428c799e26cd2a0