Fixes #7921 by setting _currentTabIndex in the RNNBottomTabsController before initialization happens.
_currentTabIndex needs to be set before the super() is called, since - [RNNBottomTabsController selectedViewController] will be called during initialization and which depends on _currentTabIndex. If not set, an initial render of the tab content with index 0 will occur when tabsAttachMode is set to "onSwitchToTab".
Fixes #7921 by setting _currentTabIndex in the RNNBottomTabsController before initialization happens.