Platform mapping-based product config

This allows us to set product variables as build settings instead
of loading them from a target's provider, which further allows us
to read product config variables in transitions.

Bug: 287539062
Bug: 269577299
Test: Presubmits
Change-Id: I8497703f706162572ceb3486240e1eb02a37f5f6
diff --git a/android/fixture.go b/android/fixture.go
index dbc3bc5..6660afd 100644
--- a/android/fixture.go
+++ b/android/fixture.go
@@ -369,7 +369,7 @@
 
 // Allow access to the product variables when preparing the fixture.
 type FixtureProductVariables struct {
-	*productVariables
+	*ProductVariables
 }
 
 // Modify product variables.