Show the bar chart detail after hiding it.

In a previous commit I made the bar chart detail text GONE when no
text was provided to avoid a crash.  But I forgot to mark it visible
when text is passed afterwards.  This fixes that bug.

Test: Create bar chart with an empty detail text, set some text, and
then see the detail text.
Test: atest com.android.settingslib.widget.BarChartPreferenceTest

Change-Id: I4a352245c3dc7b02ab3d1de21c348f2d431e95cf