[AppConfig] Make sure the consistency behaviors between languages.

[mssfang]

For the remaining consistency issue that java has in Azure/azure-sdk#2801.

Changing the strongly-typed properties after setting the value to non-valid feature flag

1. Create a feature flag
2. Set the Value to Hello world (no quotes)
3. Set some strongly typed properties

C#: Value is updated. Strongly typed properties throw on access.
Java: value is updated but don't automatically convert to the strongly-typed property value. no error throws [need to be addressed] // [Resolved] Strongly typed properties throw on access.
Python: Value is updated. Properties throw ValueError on access.
JS: Value is updated. parseFeatureFlag throws since the value can't be parsed

Accessing strongly typed properties after setting a different feature flag JSON

1. Create a feature flag
2. Set the Value to some pre-populated
3. Access strongly-typed property values

C#: Value is updated. Strongly typed properties return new values out of the parsed JSON
Java: value is updated but don't automatically convert to the strongly-typed property value. [need to be addressed]
// [Resolved] Strongly typed properties return new values out of the parsed JSON
Python: Value is updated, properties return new values out of the parsed JSON, returns None if it is not present.
JS: value is updated, parseFeatureFlag does parse the new value

Accessing value after changing strongly-typed properties.

1. Create a feature flag
2. Change a strongly-typed property
3. Does the Value reflect the change?

C#: Yes
Java: Yes
Python: Yes
JS: Yes