BTAdapter: clear discovery state on reset

[vkolotov]

reset() takes the controller down and back up, which clears
HCIHandler::currentScanType, but BTAdapter's own currentMetaScanType and
discovery_policy were left untouched. A discovery session that was running
before the reset therefore appeared still active afterwards, while the controller
was no longer scanning.

• Stop discovery_service and set DiscoveryPolicy::AUTO_OFF before the reset,
  so the keep-alive worker cannot race it.
• Clear currentMetaScanType and emit discoveringChanged once the controller
  reached the reset state, so listeners observe the implicit stop.
• Restore the previous policy if the controller did not reset, leaving a live
  discovery contract intact.
• DBTAdapter.reset() clears the Java-side scan cache for the same reason.

A new discovery session must be started explicitly after a reset.