Add ie_mixed_content endpoint.

I added an ie_mixed_content endpoint. It was a little different than ie_protected_mode, since mixed content can take 3 values (enabled, disabled, or prompt), instead of just enabled and disabled (for ie_protected_mode).

I also created a class inside of JsonCodec for the Registry Keys, instead of using "2500" and "1609".

I tried finding a place to put the common pieces (regLocation String and zone HashMap), but couldn't find a good spot.

Also, setCurrentSettingForZone(String zoneId, String newValue) are only used in tests. Perhaps the test should be updated/removed? I removed that method from ie_mixed_content, but left it for ie_protected_mode.