Changed accessibilityLabel to accessibiltyHint for CodeInput

[nickmly]

Fixes #843

According to: https://support.google.com/accessibility/android/answer/6378120?hl=en

Defining an android:contentDescription on any EditText or editable TextView may interfere with an accessibility service's ability to describe, navigate, and interact with text that a user enters within the item.

android:contentDescription is mapped to accessibilityLabel on the react native layer, so I changed it to a hint to prevent this warning from coming up

Note: This needs to be tested in Google Play Test Lab

[henrytao-me]

Tested. LGTM. When TextInput is empty, it reads the hint. When it isn't, it reads the content.

[timarney]

@nickmly let me see if I can get a link for testing with.