Add a runtime flag to Chrome OS for enabling BLE multi-advertising on ARC++ by increasing the number of BLE advertisers allowed from 1 to 4. Make the kMaxAdvertisements enum [1] a "default" max advertisements, and in the constructor of ArcBluetoothBridge check if our flag is set--in which case set the number of advertisers to 4, otherwise use the default value (1).

1. https://cs.chromium.org/chromium/src/components/arc/bluetooth/arc_bluetooth_bridge.h?l=477