[Android] Update SystemUiMode documentation to reflect edge-to-edge as the default mode and update template counter app to use edge-to-edge #150367
Description
Because edge-to-edge is becoming the default mode for apps targeting Android 15+, it will become the default SystemUiMode when Flutter targets Android 15 by default (unless the developer manually changes their target version). Thus, SystemUiMode documentation needs to be updated to reflect this fact.