Download Overlays now from the Play Store and you'll be able to key up on any of your Zello channels while you are in another app. This app is FREE!
This app is really cool. I have it set up to where I can long press the home button and a Zello PTT button will appear over top of whatever app I'm currently in. This comes in useful when you are in youtube or netflix watching a video, or while in a browser looking at a news artical or something. You would still be able to key up without having to leave the app you're in and going to Zello. You can also resize the move the button while it's on the screen to get it out of the way if need be. This is a very useful little tool.
Check out the screenshot. The widget button will display which channel you are currently in.