[fenix] For https://github.com/mozilla-mobile/fenix/issues/26884: [UI tests] Reset modified settings just before the Activity finishes
This should prevent issues with the cleanup being done earlier (even in @After)
and affecting the behavior of the Activity which is still running.
With this issue fixed I've re-enabled verifyCustomizeHomepageTest.
Because when FeatureSettingsHelper is instantiated it caches the settings
values from that instant it's important to only use one instance of it so I've
updated all tests that are using HomeActivityTestRule.
HomeActivityIntentTestRule may be updated with a similar functionality later.