Refactor prebuilt_apis.go

Improve code re-use and improve the names and data types of various vars
and functions. This makes it easier to extend it for the new use-case of
also supporting sdk extension versions on top of the current api level
versions.

This change is intended to be a noop.

Bug: 220086085
Test: m nothing (existing soong tests)
Change-Id: Icf4b320285f791e3b335f71f3c8b1ac52129dbc9