Handle legacy process event first on connection complete

[desmond-blue]

Description

This is a proposed fix for mbed-os-example-ble issue 257.

SM control block doesn't get ready when calling user-defined on_connection_complete process event, so I move the legacy process event forward.

Tested with BLE_SM/BLE_GAP/BLE_Battery/BLE_GattServer on NRF52840_DK.

Updated:
This PR only fix Error during SM::setLinkSecurity 4.
To make BLE SM example work, onAdvertisingEnd() should care if the connection is established, like this mbed-os-example-ble#265 does.

[x] Fix
[ ] Refactor
[ ] Target update
[ ] Functionality change
[ ] Docs update
[ ] Test update
[ ] Breaking change

Reviewers

Release Notes

[ciarmcom]

@desmond-blue, thank you for your changes.
@ARMmbed/mbed-os-pan @ARMmbed/mbed-os-maintainers please review.

[AGlass0fMilk]

This does not completely resolve ARMmbed/mbed-os-example-ble#257 when I build that example with this branch.

[desmond-blue]

@AGlass0fMilk You are correct, I didn't state it well.
This PR only fix Error during SM::setLinkSecurity 4.
To make BLE SM example work, onAdvertisingEnd() should care if the connection is established, like this mbed-os-example-ble#265 does.

[0xc0170]

CI started

[mbed-ci]

Test run: SUCCESS

Summary: 11 of 11 test jobs passed
Build number : 1
Build artifacts