Bluetooth: Add management command for setting LE scan parameters

The scan interval and window parameters are used for LE passive
background scanning and connection establishment. This allows
userspace to change the values.

These two values should be kept in sync with whatever is used for
the scan parameters service on remote devices. And it puts the
controlling daemon (for example bluetoothd) in charge of setting
the values.

Main use case would be to switch between two sets of values. One
for foreground applications and one for background applications.

At this moment, the values are only used for manual connection
establishment, but soon that should be extended to background
scanning and automatic connection establishment.

Signed-off-by: Marcel Holtmann <marcel@holtmann.org>
Signed-off-by: Johan Hedberg <johan.hedberg@intel.com>