Bluetooth: Host: Make bt_le_addr_is_bonded public

Some GATT services and profiles define specific behavior
based on whether the remote device is bonded or not.
The internal function, bt_addr_le_is_bonded, is the
only function to do this, but it was kept internal,
and could thus not be used for those services
without including hci_core.h.

The function has been moved to the public API
so that application can determine if a remote
address is bonded or not, and has been renamed
to not use the bt_addr namespace, but rather the
bt_le.

Signed-off-by: Emil Gydesen <emil.gydesen@nordicsemi.no>