Fix the behaviour for explicitly switching WebView provider

Before this CL when we tried to change WebView provider through the Dev
setting and failed we would then have the setting point to the provider
used before the change. With this CL we instead let the setting point to
the intended target but also switch to the package which the
WebViewUpdateService see as most fit (rather than just reverting back to
the original choice).

Add unit tests to verify we kill processes depending on WebView
packages when switching WebView provider.

Add unit tests to ensure we can recover from trying to change provider
when there are no providers available.

Bug: 27673076
Bug: 27635535
Change-Id: Ie7bb6afdf8acf4344cfb363947929c0b492775fb