Fix #7114: Drop tmp directory on launch when it exceeds a size threshold

[kylehickinson]

This replaces the webkit folder cleanup, but we could technically have both if we need

Summary of Changes

This pull request fixes #7114

Submitter Checklist:

• Unit Tests are updated to cover new or changed functionality
• User-facing strings use NSLocalizableString()
• New or updated UI has been tested across:
  • Light & dark mode
  • Different size classes (iPhone, landscape, iPad)
  • Different dynamic type sizes

Test Plan

For simulator testing:

• Get the tmp folder directory and open it in Finder
• Launch the app and verify it doesn't get removed & re-added
• Create a file in the tmp directory that exceeds the size limit (100MB). (example command to do so: $ dd if=/dev/zero of=/path/to/tmp/test.dat bs=150M count=1)
• Re-launch app and see the app size drop

For in-app testing:

• Follow steps from [WebKit] File uploaded are not cleaned up when Brave is force closed with the page open #7036 but choose a large file that exceeds 100MB, or repeat steps with small file multiple times until size exceeds 100MB
• Verify that once you exceed 100MB, the following app launch empties the tmp folder

Reviewer Checklist:

• Issues include necessary QA labels:
  • QA/(Yes|No)
  • bug / enhancement
• Necessary security reviews have taken place.
• Adequate unit test coverage exists to prevent regressions.
• Adequate test plan exists for QA to validate (if applicable).
• Issue and pull request is assigned to a milestone (should happen at merge time).