Track WebViewCompatibility reflection failure and fallback to legacy gracefully

[aforcier]

Users with non-stock API<19 devices may experience a reflection failure here. This is possible if their OEM has tampered with WebView internals. We're not aware of any such devices at the moment, but anything is possible in the Android landscape.

If this happens, we should load the legacy editor, and make sure we're tracking the error so we can possibly add a fix for those devices. We could disable the visual editor entirely, but I'm concerned with how we would notify the user that they can use the visual editor again when we fix the issue for their device.

Relevant TODO: EditorWebViewCompatibility.java#L107