Change API for killing app-depentents to also kill main app.

In the case where we switch WebView provider we also want to kill the
provider itself in the cases where we want to kill its dependents. This
so that the provider's processes are consistently loading code in the
same way (i.e. either all processes use the webview implementation, or
none of them do).

Bug: 25338573

Change-Id: I2baf7ce1204fb6312a6ad10fa006b789c8ece176