Expose ProductVariables from TestConfig

In preparation for unexporting ProductVariables, explicitly return a
pointer to the structure from TestConfig / TestArchConfig.

Bug: 76168832
Test: m blueprint_tools
Change-Id: Iccfb4c912f8e0ee3f620cc1ee00f0cdc5cba7735
Merged-In: Iccfb4c912f8e0ee3f620cc1ee00f0cdc5cba7735
(cherry picked from commit 674dc7f7)