Update the Online search Engine key to "selected_search_engine".

Test steps:
1. Execute adb shell settings put secure selected_search_engine <search_engine_package_name> (Ex: com.duckduckgo.mobile.android)
2. Install the app and verify the search widget on launcher is updated.
3. Uninstall the app and verify the search widget is replaced by
   placeholder

When we change the search engine setting after the app is installed , we need to force-stop launcher app for the new settings to take effect.

Bug: 333029477
Flag: EXEMPT bugfix
Test: Manual
Change-Id: If127dcc87c5278a98177b83514cddb09a4124223