Add some API implementations

This adds initial implementations of StartDiscovery and CancelDiscovery,
and CreateBond. At this point the implementations are focused on making
a HID use case work and still missing other case handling.

Bug: 188718204
Bug: 188718859
Tag: #floss
Test: manual - build floss on Linux

Change-Id: I7a218feb902d27397ac7c35a3fe5ad98cc0d2759