Bluetooth: Add support for persistent pairing keys storage

Integrate the bt_keys submodule with bt_settings. Add a new
bt_keys_store() API to write keys to flash, and extend the existing
bt_keys_clear() to remove the keys from flash.

Along with this, add some helpers for genrating settings key values
containing a bluetooth address, as well as for decoding them to get
the binary bt_addr_le_t.

Signed-off-by: Johan Hedberg <johan.hedberg@intel.com>