Yes. Withdrawal functionality is not available for mobile yet. Both for Android and iOS. It’s in the works.
The default behavior is to use BAT you earned to support your favorite creators as a “thank you” for their contents. 
For now, you can kept it in your brave:rewards wallet until the feature is implemented.
But do note, if you uninstall Brave, you’ll lost your browser data including your wallet data.
Thanks for your patience.