Fix WebView loading logic to allow changing provider if current disabled

Add an XML tag declaring whether a package can be used without
explicitly being chosen by the user (it is available-by-default).

Change the WebView loading logic to either
1. load a user-chosen and enabled package or
2. load an enabled and available-by-default package or
3. any package that is valid

Bug: 26400585
Change-Id: I8de253c1687e3cc7961184c2d770d4e385d6187a