If you own a device, you're entitled to a copy of all GPL software that came with it, including some build tools. However, if the diminished battery performance comes from some kind of firmware update for an embedded microcontroller, you're probably out of luck there, unless that controller also runs GPL software.
It's possible the changes you're looking for are already listed on one of these repos: https://android.googlesource.com/device/google/sunfish-sepol... https://android.googlesource.com/device/google/sunfish-kerne... https://android.googlesource.com/device/google/sunfish/
(sunfish is the codename for the Pixel 4a)