Install another TV launcher app (I like Simple TV Launcher). Once installed, use Android dev tool on a remote laptop/workstation (platform-tools, adb.exe) to remotely connect to the Shield device and disable Google TV. The command to disable Google TV launcher is something like 'adb shell pm disable-user --user 0 google.android.tv'

Once Google TV Launcher is disabled, Shield picks the next installed Launcher on your system as the default TV Launcher. Be aware that most 3rd party TV launchers aren't very configurable or polished but will launch your other apps. Maybe try a few different ones before disabling Google TV Launcher.