QBluetoothDeviceDiscoveryAgent - timeout on OS X/iOS

On OS X we have to first make Classic device scan and then LE device scan.
Shorten Classic discovery from 20s to 15s.

Change-Id: I653b2d9f6c911bdf09c900735273bd6d1e95b078
Reviewed-by: Alex Blasche <alexander.blasche@theqtcompany.com>