Add cow_api_test to presubmit.

These are unit tests for important VAB functionality. They are very
quick to run and should be included in presubmit testing.

Bug: N/A
Test: th green
Change-Id: I02e24c76df365e9fb2d68f904e930dce60b9bdaf

fs_mgr/TEST_MAPPING

@@ -17,6 +17,9 @@
     },
     {
       "name": "libsnapshot_fuzzer_test"
+    },
+    {
+      "name": "cow_api_test"
     }
   ]
 }

fs_mgr/libsnapshot/Android.bp

@@ -479,6 +479,9 @@ cc_test {
         "libgtest",
         "libsnapshot_cow",
     ],
+    test_suites: [
+        "device-tests"
+    ],
     test_min_api_level: 30,
     auto_gen_config: true,
     require_root: false,