Update cross-signing status ui after activation in settings

[NicolasBuquet]

When activating cross-signing in Security settings, either directly or by activating Secure Backup, the cross-signing status is not updated in UI.

If you leave Security Settings and open it back right away, the cross-signing status is updated.

Update it right after activation without having to leave the screen.

Pull Request Checklist

• [X ] I read the contributing guide
• [ ] UI change has been tested on both light and dark themes, in portrait and landscape orientations and on iPhone and iPad simulators
• [ ] Accessibility has been taken into account.

• [X ] Pull request is based on the develop branch

• [X ] Pull request contains a changelog file in ./changelog.d
• [X ] You've made a self review of your PR
• [ ] Pull request includes screenshots or videos of UI changes
• [X ] Pull request includes a sign off

[codecov[bot]]

Codecov Report

Attention: 2 lines in your changes are missing coverage. Please review.

Comparison is base (d50b82a) 12.37% compared to head (be4fbc3) 12.03%. Report is 25 commits behind head on develop.

Files	Patch %	Lines
...Modules/Settings/Security/SecurityViewController.m	0.00%	2 Missing :warning:

Additional details and impacted files

@@             Coverage Diff             @@
##           develop    #7714      +/-   ##
===========================================
- Coverage    12.37%   12.03%   -0.34%     
===========================================
  Files         1650     1650              
  Lines       163869   163920      +51     
  Branches     67267    67277      +10     
===========================================
- Hits         20277    19727     -550     
- Misses      142927   143533     +606     
+ Partials       665      660       -5     

Flag	Coverage Δ
uitests	55.03% <ø> (+0.06%)	:arrow_up:
unittests	5.86% <0.00%> (-0.36%)	:arrow_down:

Flags with carried forward coverage won't be shown. Click here to find out more.

:umbrella: View full report in Codecov by Sentry.
:loudspeaker: Have feedback on the report? Share it here.