recompute care_map.pb rather than passing it through

In short (typing on my phone, can go into more detail if needed), I ran into an issue where the care_map.pb entry had drifted from what the actual partition size was. This caused a failure to boot after OTA update. Currently avbroot simply passes the existing care_map.pb through when patching an OTA package. My fix waa to recompute the care_map.pb after the payload.bin had been packed and before the final zip assembled. I can submit a PR, but it contains an AI-authored re-implementation of AOSPs code to generate the care_map.pb. Your call, I can submit that if you like. As far as I can tell, it's unique and doesn't exist elsewhere / is not a violation of any IP rights. Not sure I've even pushed it to my own repo, tbh. Let me know if you're interested in checking it out. Otherwise, here's a bug report :)