MV3: Add flag to disable MV3 service worker keep alive code

[danjm]

The saveTimestamp function is called at an interval in background.js to prevent the service worker from dying. We should put this behind a flag, so that we can disable this behaviour easily.

Once this behaviour is disabled, we should test how the extension behaves if/when dapps communicate with the extension after the service worker has been termined

[danjm]

To complete this task:

• this code needs to be in an if block: https://github.com/MetaMask/metamask-extension/blob/develop/app/scripts/background.js#L287-L292
• the if condition should be a boolean property enableMV3TimestampSave
• that property should be managed by the preferences controller, and should default to true
• the property should be controlled by a toggle in settings in the new "Developer Options" section, that was introduced in this PR https://github.com/MetaMask/metamask-extension/pull/22382 (so the toggle will not be visible on production builds)

Once the above is done, we should able to toggle the functionality on and off in MV3 dev builds, and see the timestamp saving when toggled on and not saving when toggled off

[metamaskbot]

Missing release label release-11.17.0 on issue. Adding release label release-11.17.0 on issue, as issue is linked to PR #24326 which has this release label.

[metamaskbot]

Missing release label release-11.16.6 on issue. Adding release label release-11.16.6 on issue, as issue is linked to PR #24326 which has this release label.