update_verifier: Support verifying product partition.

We have added the support for building /product partition in build
system (the CL in [1]), where /product is an optional partition that
contains system files. This CL adds the matching support if /product
needs to be verified during A/B OTA (i.e. listed in care_map file).

[1]: commit b7735d81054002961b681f4bdf296d4de2701135,
https://android-review.googlesource.com/c/platform/build/+/598454

Bug: 63974895
Test: Run update_verifier test on walleye.
Change-Id: Ia1c35e9583b8e66c98a4495b1f81a5ea7e65036f

tests/component/update_verifier_test.cpp

@@ -46,7 +46,6 @@ TEST_F(UpdateVerifierTest, verify_image_smoke) {
     return;
   }
 
-  // The care map file can have only two or four lines.
   TemporaryFile temp_file;
   std::string content = "system\n2,0,1";
   ASSERT_TRUE(android::base::WriteStringToFile(content, temp_file.path));
@@ -58,7 +57,7 @@ TEST_F(UpdateVerifierTest, verify_image_smoke) {
 }
 
 TEST_F(UpdateVerifierTest, verify_image_wrong_lines) {
-  // The care map file can have only two or four lines.
+  // The care map file can have only 2 / 4 / 6 lines.
   TemporaryFile temp_file;
   ASSERT_FALSE(verify_image(temp_file.path));