In the app "Bitcoin wallet" from " bitcoin wallet developers" you can choose to backup your wallet. It will store this backup on your sd-card. But how can you restore it on another phone? I saved the backup on my PC and send it to my other phone, but he can't open the file and I don't know how to restore the wallet.
you have in the wallet on the right top the 3 points. click on it. click on "security" (or something like that, on my phone the language is not english). in my case i have several options including "save wallet" and "restore wallet" (or something like that). when you click the restore-option you should be able to choose your backup if the app finds your backup (in my case it finds my test-backup in the downloadfolder on the sd-card for example).