Use Config/DeviceConfig functions to access ProductVariables

An upcoming change will stop exporting ProductVariables from Config, so
switch to using existing accessor functions, and add more when they're
missing.

Bug: 76168832
Test: out/soong/build.ninja is identical
Change-Id: Ie0135bdbd2df3258ef3ddb53e5f8fc00aa9b97f7
Merged-In: Ie0135bdbd2df3258ef3ddb53e5f8fc00aa9b97f7
(cherry picked from commit 3fb1faee)