Floss: Also listen for BlueZ suspend API

For BlueZ to communicate with powerd, it is easier to use btmanagerd to
reuse protobuf and powerd state machine logic. This adds btmanagerd to
also listen to BlueZ emitting Suspend API-compliant object and relays
powerd events there. No special handling is needed for BlueZ with the
assumption that only one of BlueZ (bluetoothd) or Floss (btadapterd) can
run at a time.

Bug: 231741170
Tag: #floss
Test: Manual - Build Floss on Linux

Change-Id: I2edd1024c02b8120f2a8cfcc0efb3737bcfc7671