[BluetoothLEHelper] ObservableGattCharacteristics improvements (respect properties)

[GeertvanHorrik]

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Current behavior

The ObservableGattCharacteristics does not respect the properties when calling methods.

Expected behavior

Respect the bluetooth specs.

Potential fix

These fixes might contain pseudo code:

• ReadValueAsync

if (!Enum<GattCharacteristicProperties>.Flags.IsFlagSet(_characteristic.CharacteristicProperties, GattCharacteristicProperties.Read))
{
    Log.Debug($"[{this}] Reading is not supported");
    return null;
}

• SetIndicateAsync

if (!Enum<GattCharacteristicProperties>.Flags.IsFlagSet(_characteristic.CharacteristicProperties, GattCharacteristicProperties.Indicate))
{
    Log.Debug($"[{this}] Indicate is not supported");
    return false;
}

• SetNotifyAsync

if (!Enum<GattCharacteristicProperties>.Flags.IsFlagSet(_characteristic.CharacteristicProperties, GattCharacteristicProperties.Notify))
{
    Log.Debug($"[{this}] Notify is not supported");
    return false;
}