Collapse CurrencySecondaryStatus when no text is present in the text …

…block (#467)

Fixes #313
In Scan/Item mode, Narrator focus navigates to hidden element “No next item” after “Update rates” link in "Currency Converter" window #313

Description of the changes:
Adds an x:Name to the CurrencySecondaryStatus text block
Adds a NormalCurrencyStatus visual state to the CurrencySecondaryStatusStates
Adds functionality to the CurrencySecondaryStatusStates to show or hide the CurrencySecondaryStatus text block.

How changes were validated:
Verified that the textblock is not visible in the accessibility tree via inspect.exe from the windows sdk.
Verified that Narrator also does not stop on the block when in scan mode.
Verified that the textblock is visible in the accessibility tree and read out in Narrator when the ChargesMayApplyCurrencyStatus or FailedCurrencyStatus viewstates are set.

src/Calculator/Views/UnitConverter.xaml

@@ -615,8 +615,7 @@
                                          x:Uid="RefreshButtonText"
                                          Foreground="{ThemeResource SystemControlHyperlinkBaseHighBrush}"
                                          Click="CurrencyRefreshButton_Click"/>
-                        <TextBlock Margin="0,7,0,0" Style="{ThemeResource CaptionTextBlockStyle}">
-                            <Run x:Name="Spacing" Text="&#x200A;"/>
+                        <TextBlock Margin="3,7,0,0" Style="{ThemeResource CaptionTextBlockStyle}">
                             <Run x:Name="CurrencySecondaryStatus"
                                  FontWeight="SemiBold"
                                  Text=""/>