Fix disabled WebView packages being shown as enabled in Dev Setting.

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