`DropdownTextView` does not always function correctly

[Cliabhach]

This corresponds to the problems we saw in #358 and #357 , under ticket AIC-646.

Put simply, if we define

<edu.artic.view.DropdownTextView
    android:id="@+id/whatever"
    android:layout_width="match_parent"
    android:layout_height="wrap_content"
    android:visibility="gone"
    />

then RxJava2 calls such as this one sometimes fail to trigger. While we have not been able to calculate a precise incidence rate the Samsung Galaxy S8+ is particularly susceptible. Our current logic inverts the logic (that is, show the views by default and only hide them if derived text is empty), and this seems to work.

@agrosner do you have any thoughts? Perhaps the show binding is being set up too late or disposed of too early?