[iOS] - Fix popover blocking input when backgrounding the app.

[Brandon-T]

Summary

• Fix the Leo Menu popover blocking the app from working when backgrounding while it's active since it's not properly removed from the screen when the system automatically dismisses it.

Resolves brave/brave-browser#39109

Submitter Checklist:

• I confirm that no security/privacy review is needed and no other type of reviews are needed, or that I have requested them
• There is a ticket for my issue
• Used Github auto-closing keywords in the PR description above
• Wrote a good PR/commit description
• Squashed any review feedback or "fixup" commits before merge, so that history is a record of what happened in the repo, not your PR
• Added appropriate labels (QA/Yes or QA/No; release-notes/include or release-notes/exclude; OS/...) to the associated issue
• Checked the PR locally:
  • npm run test -- brave_browser_tests, npm run test -- brave_unit_tests wiki
  • npm run presubmit wiki, npm run gn_check, npm run tslint
• Ran git rebase master (if needed)

Reviewer Checklist:

• A security review is not needed, or a link to one is included in the PR description
• New files have MPL-2.0 license header
• Adequate test coverage exists to prevent regressions
• Major classes, functions and non-trivial code blocks are well-commented
• Changes in component dependencies are properly reflected in gn
• Code follows the style guide
• Test plan is specified in PR before merging

After-merge Checklist:

• The associated issue milestone is set to the smallest version that the
  changes has landed on
• All relevant documentation has been updated, for instance:
  • https://github.com/brave/brave-browser/wiki/Deviations-from-Chromium-(features-we-disable-or-remove)
  • https://github.com/brave/brave-browser/wiki/Proxy-redirected-URLs
  • https://github.com/brave/brave-browser/wiki/Fingerprinting-Protections
  • https://github.com/brave/brave-browser/wiki/Brave%E2%80%99s-Use-of-Referral-Codes
  • https://github.com/brave/brave-browser/wiki/Web-Compatibility-Exceptions-in-Brave
  • https://github.com/brave/brave-browser/wiki/QA-Guide
  • https://github.com/brave/brave-browser/wiki/P3A

Test Plan:

[kjozwiak]

Verification PASSED on iPhone 11 running iOS 17.5.1 via the following build(s):

Brave | 1.69.83 Chromium: 127.0.6533.26 (Official Build) nightly (64-bit) 
--- | ---
Revision | 82eef43ce82a...
OS | iOS

Using the STR/Cases outlined via brave/brave-browser#39109 (comment), ensured that the Leo settings menu was still working as expected when switching between other applications while the Leo settings menu was visible as per the following:

RPReplay_Final1720222037.MP4

Created/Ran into brave/brave-browser#39595 while verifying the above for uplift.