Gets the horizontal positioning of tabs at the top of this TabNavigator container.
Possible values are: left, center, and right. The default value is left.
If the value is left, the left edge of the first tab is aligned with the left edge of the TabNavigator container. If the value is right, the right edge of the last tab is aligned with the right edge of the TabNavigator container. If the value is center, the tabs are centered on the top of the TabNavigator container.
To see a difference between the alignments, the total width of all the tabs must be less than the width of the TabNavigator container.