floss: Add bond remove and cancel to btclient

Refactors the bond command to take "add|remove|cancel" and wires the
command to take any of those actions by calling the correct dbus
methods.

Bug: 196887009
Tag: #floss
Test: btclient on ChromeOS
Change-Id: Icfee9edba495f0e13791414485cf124d6b2d952a