Update persistent WebView packages setting only when user changes it.

To ensure that we don't permanently change WebView implementation if the
current package is temporarily uninstalled (e.g. when being replaced) we
don't update our persistent setting unless the user explicitly changes
WebView implementation (and on boot!).

Unfortunately this will means that the Dev Setting for changing WebView
implementation will work in a slightly less intuitive way. The
persistent setting is now persistent across uninstalls and installs.
I.e. the Dev Setting shows the current WebView implementation though
that could differ to the value chosen by the user since the package
chosen by the user could be uninstalled or disabled. In this case
installing/enabling that package would again make the Dev Setting point
to it.
However, as a compromise, we do change the setting at boot so that if
the currently chosen package is not valid we will change the setting so
that it points to the package we currently use instead.

Also ensure we only use WebView packages that are available-by-default
if no WebView packages are enabled.

Add unit test to ensure that if a user-chosen provider is uninstalled we
switch back to using that provider when it is installed again.
Add unit test to ensure we switch user-chosen provider at boot if the
chosen one is uninstalled.

Bug: 27673076
Change-Id: Icd27ae302798ebf695b9ef4bd4d5fd47fe4be02c