My philips 4k tv keeps most used apps in memory like a charm since android 8 update. Netflix and youtube RARELY reload after watching tv or the tv was switched off. And it's not a high end model tv, it has a meere 1.5GB ram and some Mediatek cpu so nothing to write home about.
Sometimes if I turn off the tv while playing youtube video, next time i start the tv and tap on youtube it resumes from the exact same spot.
Your best option is a foreground service with a permanent notification. My app actually needs and does this (VPN app)
I have a shield as well, I use one app that needs a different one running for it to work. I just open the assistant app and press the home button to keep it going, works fine
I know that you can change your shield to never close out an app in the developer settings. Though I have never done it. I have not found a need for it.
On my Shield I often watch something on YouTube, pause it in the middle, switch on kodi to watch a movie or TV series episodes, put the shield on standby, and continue watching YouTube the next day without issue. YouTube never closed itself while in background...
Short answer: NO.
Longer answer: There is a process/service on the device that purposely kills applications in the background to free up resources for apps that are currently in the foreground or for apps that will launch and go foreground. The more memory/resources your device has, the less pressure there is for this process/service to engage in this kind of housekeeping - but it will still happen.