Align the "Open activity" UI style with usual accessibility service "Use service"

Root Cause: "Open activity" Preference is not part of the widget SettingsMainSwitchBarPreference, so its UI style did not be inherited.

Solution: "Open activity" has different concept from switch bar, we need to manual set its bacground to comply with the SettingsLib style.

Bug: 207067203
Test: manual test
Change-Id: I66f63f6952e5c4770509febc0e53eca6dc6eedb9