Support Agassi (Search Engine choice) in Android R

Test:
1) set your preferred search provider using `adb shell settings put global SEARCH_PROVIDER_PACKAGE_NAME <value>`.(eg: com.duckduckgo.mobile.android)
2) install the package name and confirm search widget on launcher is updated
3) uninstalled the package and confirm widget is replaced with placeholder on launcher.

Bug:137653564
Change-Id: I8904d5915a8c03ea6068fb5950af15ca786b8ad7