Duplicated functionality in GapAdvertisingParams and GapScanningParams

GapAdvertisingParams and GapScanningParams use `MSEC_TO_ADVERTISEMENT_DURATION_UNITS()` and `MSEC_TO_SCAN_DURATION_UNITS()` respectively to convert from milliseconds to units of 0.625. However, these two functions do exactly the same operation:

```
return (durationInMillis * 1000) / UNIT_0_625_MS;
```

Therefore, this functionality is duplicated. Perhaps it should be included in a common location such as blecommon.h or BLEProtocol? These conversions are necessary because those are the units that the BLE specification uses, they are not specific to advertising or scanning params.

**NOTE:** The same goes for `UNIT_0_625_MS` in [here](https://github.com/ARMmbed/ble/blob/develop/ble/GapAdvertisingParams.h#L80) and [here](https://github.com/ARMmbed/ble/blob/develop/ble/GapScanningParams.h#L35).