The problem is that there is no standard USB code that maps to KEYCODE_WINDOW in most android versions.
The code 171 is an Android-specific value and not something that can be sent directly by a Bluetooth or usb keyboard.
I fixed this in https://android-review.googlesource.com/c/platform/frameworks/base/+/1365553, but not many devices are using that patch yet.
@tmm1
Request for a little education for me 
The letter 'p' works from my keyboard....how's that being mapped?
I imagine that Channels is receiving the KEYCODE associated with "P", and initiating the window event. This is a feature of Channels, not part of the Android system. (That's why it's not working with the CCGTV: because the windowing code is either not present or changed from previous Android versions.)
Thanks for the reply.....makes sense.
Then it should be possible for Channels to map that to another keycode as well....say another function 'F' key that is already present in the harmony device profile and we should be all set.
If you add a device like "Windows computer" to your Harmony hub, you can have your remote send a "P" from the other device, and it'll go over bluetooth, too. You are not limited to a single device when using the hub. (I've mixed Fire TV and Shield keys on a single remote to get the features I want.)
There isn't anything additional Channels needs to do. You just need to be a bit more creative in how you setup your Harmony Hub devices.
Except I'm a bit constrained by the 8 device limit....
Thanks for the post. I got it working with the hub and companion remote. Last night I could only get it to work with the app but finally got it working with the remote too.
I bought the Harmony Ultimate to pair with the hub I got with the Companion Remote which is limited to 8 devices. Adding the Ultimate jumps the device limit to 15. It was on sale at the time. The button layout isn't nearly as good so I rarely use it but the added devices was worth the cost.
Is there any tricks after going into PIP mode, and you don't find anything better to watch, or start a recording on another channel, to simply go back to full screen? Yes, if you go to now playing, and find the show you are watching, or find the recording you are watching, that will get you back to fullscreen.
Am I missing something? Has this been requested? Should I add this to feature requests?
If you go back to the PIP window there is a button to maximize the window again I think..
How do you go back to the pip window? What button is that?
It says hold home to control PIP but I can't figure out what that means.
It’s referring to the home button on the Apple TV remote. Have you tried that?
No. I am using a Firestick 4K.
Ahh ok. My mistake. It sure on that client, sorry.
I've added this to the latest beta. It can both start and stop PIP (as can the 'p' key).
This is working great! Thank you!
@tmm1
Having to double select return to full screen from the guide with PiP active is REALLY annoying (Android client). Please put it back the way it was as a single select.
Can you add to turn on pip automatically when you click on the menu button on Fire TV remote, it appears automatic when the guide appears that way I don't have to grab my ipad and open the Fire TV app to bring on the pip to view the guide. This would be great thanks.
Note: Any new client builds you update are only the Alpha or Beta or both being updated.
By "the way it was", do you mean when you couldn't get out of PIP by selecting the "p" button? I would say the Resume Playing/Start from the Beginning option after getting out of PIP, is not needed, but the "p" option to get out of PIP certainly is.
Another option I would suggest would be when the Resume Playing/Start from the Beginning option pops up, after getting out of PIP with the "p" button, if you don't select one of those options within 5 seconds, the box simply goes away! That way you don't have to select anything, you still get the option, and there is no delays by simply going back to fullscreen. As it stands now, selecting "Resume Playing" causes a buffering delay which is very annoying.
I do appreciate the work on this by the developers.