WebView packages that cannot be chosen as WebView implementation should
be shown as disabled in the 'WebView Implementation' Dev Setting.
Simplify the logic in DefaultAppInfo a little bit to avoid
special-casing for the WebView Dev setting.
Also add tests to ensure we are correctly disabling WebView packages in
the Dev setting and showing their disabled-reasons.
Bug: 35707106
Test: 'make RunSettingsRoboTests'
Test: Ensure disabled WebView packages show up as being disabled in the
'WebView Implementation' Dev Setting.
Change-Id: Ic2f5265c3451b6396e55b7bc29689c3d889ddb5e