Revert the change to allow overriding any game modes through cmd

Since the game modes read from getAvailableGameModes API will not
include those in override configs, we should only allow overriding
game modes predefined for a game here.

Bug: b/240335717
Test: adb shell cmd game set --mode 2 --downscale 0.3 $PKG
Change-Id: I89d792195cf99577b0d59ec73b0a94010326d6f7