Fix test

Use environment to get the right API level instead of hardcoding it in
the check

Author: rlazo

firebase-inappmessaging/src/test/java/com/google/firebase/inappmessaging/internal/ApiClientTest.java

@@ -49,6 +49,7 @@
 import org.mockito.Mock;
 import org.mockito.MockitoAnnotations;
 import org.robolectric.RobolectricTestRunner;
+import org.robolectric.RuntimeEnvironment;
 import org.robolectric.annotation.Config;
 
 @RunWith(RobolectricTestRunner.class)
@@ -210,7 +211,7 @@ public void getFiams_signalsContainsPlatformVersion() {
         fetchEligibleCampaignsRequestArgcaptor.getValue().getClientSignals();
 
     // sdk version set in roboelectric annotation above
-    assertThat(clientSignals.getPlatformVersion()).isEqualTo(String.valueOf(Config.OLDEST_SDK));
+    assertThat(clientSignals.getPlatformVersion()).isEqualTo(String.valueOf(RuntimeEnvironment.getApiLevel()));
   }
 
   @Test