MediaMetrics: Add Elem getter to Item

And minor edits.

Test: atest mediametrics_tests
Bug: 138583596
Change-Id: I9161f98f4c17516ea04fc143eb1ccbd656f17796
diff --git a/services/mediametrics/tests/mediametrics_tests.cpp b/services/mediametrics/tests/mediametrics_tests.cpp
index 55ce82b..79cb2af 100644
--- a/services/mediametrics/tests/mediametrics_tests.cpp
+++ b/services/mediametrics/tests/mediametrics_tests.cpp
@@ -262,27 +262,32 @@
           int32_t i32;
           ASSERT_TRUE(prop.get(&i32));
           ASSERT_EQ(1, i32);
+          ASSERT_EQ(1, std::get<int32_t>(prop.get()));
           mask |= 1;
       } else if (!strcmp(name, "i64")) {
           int64_t i64;
           ASSERT_TRUE(prop.get(&i64));
           ASSERT_EQ(2, i64);
+          ASSERT_EQ(2, std::get<int64_t>(prop.get()));
           mask |= 2;
       } else if (!strcmp(name, "double")) {
           double d;
           ASSERT_TRUE(prop.get(&d));
           ASSERT_EQ(3.125, d);
+          ASSERT_EQ(3.125, std::get<double>(prop.get()));
           mask |= 4;
       } else if (!strcmp(name, "string")) {
           std::string s;
           ASSERT_TRUE(prop.get(&s));
           ASSERT_EQ("abc", s);
+          ASSERT_EQ(s, std::get<std::string>(prop.get()));
           mask |= 8;
       } else if (!strcmp(name, "rate")) {
           std::pair<int64_t, int64_t> r;
           ASSERT_TRUE(prop.get(&r));
           ASSERT_EQ(11, r.first);
           ASSERT_EQ(12, r.second);
+          ASSERT_EQ(r, std::get<decltype(r)>(prop.get()));
           mask |= 16;
       } else {
           FAIL();