Add hidden-answer appearance for string with external app

[lognaturel]

We recently added the hidden-answer appearance for barcode. We've also had a request for the same appearance for external apps that return strings because those external apps can return very long/ugly values that are intended to be processed in some way before being shown to users.

We typically start a forum thread to consider new feature ideas but this feels like a clear enough extension of existing work that I think we can just make it so.

For ExStringWidgetTest:

    @Test
    public void whenPromptHasHiddenAnswerAppearance_answerIsNotDisplayed() {
        String appearance = "ex:my.cool.app(a_param=1,another=3) " + Appearances.HIDDEN_ANSWER;
        FormEntryPrompt prompt = new MockFormEntryPromptBuilder(promptWithAppearance(appearance))
                .withAnswer(new StringData("original contents"))
                .build();
        WidgetTestActivity widgetTestActivity = QuestionWidgetHelpers.widgetTestActivity();
        ExStringWidget widget = new ExStringWidget(widgetTestActivity, new QuestionDetails(prompt),
                new FakeWaitingForDataRegistry(), stringRequester);
        // Check initial value is not shown
        assertThat(widget.binding.widgetAnswerText.getVisibility(), equalTo(View.GONE));
        assertThat(widget.getAnswer(), equalTo(new StringData("original contents")));
        // Check updates aren't shown
        widget.setData("updated contents");
        assertThat(widget.binding.widgetAnswerText.getVisibility(), equalTo(View.GONE));
        assertThat(widget.getAnswer(), equalTo(new StringData("updated contents")));
    }