I think a system to verify the integrity of the operating system and make the user aware of any changes is a Good Thing. Of course, the user should be in control of what signing keys are trusted and who else gets access to that information.

Instead, what Google has done is allowed app developers to check that the user isn't doing anything surprising - especially unprofitable things like blocking ads or spoofing tracking data. Since Google profits from ads and tracking, I must assume a significant part of their motivation is to make unprofitable behavior inconvenient enough most people won't do it.