[FIAM] Replace hardcoded SDK version with Config.OLDEST_SDK (#7059)

Tests will use the Config.OLDEST_SDK instead of a hardcoded SDK if the
corresponding hardcoded SDK would be below our intended minSdk.

I'll also need to re-evaluate whether these tests are necessary to keep
around, or if they are irrelevant when bumping the minSdk.

Related to cl/770033566

---------

Co-authored-by: google-labs-jules[bot] <161369871+google-labs-jules[bot]@users.noreply.github.com>

Author: rlazo

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

@@ -43,7 +43,7 @@
 import org.robolectric.junit.rules.ExpectedLogMessagesRule;
 
 @RunWith(RobolectricTestRunner.class)
-@Config(manifest = Config.NONE, sdk = 22, qualifiers = "es")
+@Config(manifest = Config.NONE, sdk = Config.OLDEST_SDK, qualifiers = "es")
 public class AnalyticsEventsManagerTest {
 
   @Rule public final ExpectedLogMessagesRule logged = new ExpectedLogMessagesRule();

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

@@ -49,10 +49,11 @@
 import org.mockito.Mock;
 import org.mockito.MockitoAnnotations;
 import org.robolectric.RobolectricTestRunner;
+import org.robolectric.RuntimeEnvironment;
 import org.robolectric.annotation.Config;
 
 @RunWith(RobolectricTestRunner.class)
-@Config(manifest = Config.NONE, sdk = 22, qualifiers = "es")
+@Config(manifest = Config.NONE, sdk = Config.OLDEST_SDK, qualifiers = "es")
 public class ApiClientTest {
 
   public static final String TIME_ZONE = "Europe/London";
@@ -210,7 +211,8 @@ public void getFiams_signalsContainsPlatformVersion() {
         fetchEligibleCampaignsRequestArgcaptor.getValue().getClientSignals();
 
     // sdk version set in roboelectric annotation above
-    assertThat(clientSignals.getPlatformVersion()).isEqualTo("22");
+    assertThat(clientSignals.getPlatformVersion())
+        .isEqualTo(String.valueOf(RuntimeEnvironment.getApiLevel()));
   }
 
   @Test