Safely clear SnackBack and BottomSheet placeholders

Correct one stocks demo bug: displaying the stock symbol viewer (single tap on a row) while a BottomSheet or SnackBar was up, would assert.

Thep SnackBack and BottomSheet placeholder keys are cleared when they're obscured by a route with an opaque OverlayEntry. So in that case there's no need to try and clear the placeholder's child again.