This commit adds the following:
Passkey pairing - passkey is displayed on watch
Swipe down to clear passkey screen
Connection encryption
Connection bonding
Automatic reconnects to a bonded peripheral
Trusted device on Android
Note that persisting the bond between reboots is NOT included in
this commit. Therefore, rebooting the watch will cause reconnect failures.
You must delete the bond from the phone to reconnect/pair.
Always returning 0 (when `MYNEWT_VAL_BLE_LL_RFMGMT_ENABLE_TIME` is not defined), rather than a time near to the current tick, causes an issue in at least one place: `ble_ll_adv_sm_start()`, where the calculation of `delta` overflows when the system timer is at 0x80000000 or above -- causing an incorrect, huge adjustment to be made to the scheduled time, ultimately stopping adverts from being sent.
