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 674dc7f7f0)

java/java_test.go

@@ -60,7 +60,7 @@ func testConfig(env map[string]string) android.Config {
 		env["ANDROID_JAVA8_HOME"] = "jdk8"
 	}
 	config := android.TestArchConfig(buildDir, env)
-	config.ProductVariables.DeviceSystemSdkVersions = &[]string{"14", "15"}
+	config.TestProductVariables.DeviceSystemSdkVersions = &[]string{"14", "15"}
 	return config
 
 }