[Common] Replace hardcoded SDK version with Config.OLDEST_SDK (#7043)

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/769913180

---------

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

Author: rlazo

firebase-common/data-collection-tests/src/test/java/com/google/firebase/DataCollectionPreNDefaultDisabledTest.java

@@ -20,15 +20,15 @@
 import static com.google.firebase.DataCollectionTestUtil.withApp;
 
 import android.content.SharedPreferences;
-import androidx.test.ext.junit.runners.AndroidJUnit4;
 import com.google.firebase.internal.DataCollectionConfigStorage;
 import org.junit.Test;
 import org.junit.runner.RunWith;
+import org.robolectric.RobolectricTestRunner;
 import org.robolectric.annotation.Config;
 import org.robolectric.annotation.LooperMode;
 
-@RunWith(AndroidJUnit4.class)
-@Config(sdk = 21)
+@RunWith(RobolectricTestRunner.class)
+@Config(sdk = Config.OLDEST_SDK)
 public class DataCollectionPreNDefaultDisabledTest {
 
   @Test