Add hidden-answer appearance for string with external app

[grzesiek2010]

Closes #6572

Why is this the best possible solution? Were any other approaches considered?

There’s not much to discuss here. We hide the answer when the new appearance is used, similar to how it works with the barcode widget.

How does this change affect users? Describe intentional changes to behavior and behavior that could have accidentally been affected by code changes. In other words, what are the regression risks?

It simply adds a new appearance, allowing us to focus primarily on testing it to ensure answers are hidden.

Do we need any specific form for testing your changes? If so, please attach one.

Any form with external widgets (string/integer/decimal) and the new appearance. I've used:
external_widgets_hidden.xlsx
external_widgets.xlsx
which work with CollectAnswersProvider

Does this change require updates to documentation? If so, please file an issue here and include the link below.

getodk/docs#1922

Before submitting this PR, please make sure you have:

• added or modified tests for any new or changed behavior
• run ./gradlew connectedAndroidTest (or ./gradlew testLab) and confirmed all checks still pass
• added a comment above any new strings describing it for translators
• added any new strings with date formatting to DateFormatsTest
• verified that any code or assets from external sources are properly credited in comments and/or in the about file.
• verified that any new UI elements use theme colors. UI Components Style guidelines