Update references to ShadowBaseLooper to use ShadowLooper

ShadowBaseLooper is deprecated and will be removed in a forthcoming CL.

Tested:
    TAP --sample ran all affected tests and none failed
    http://test/OCL:391922969:BASE:391896312:1629439874303:285a1989
PiperOrigin-RevId: 392647041

library/core/src/test/java/com/google/android/exoplayer2/offline/DownloadHelperTest.java

@@ -17,7 +17,7 @@ package com.google.android.exoplayer2.offline;
 
 import static com.google.common.truth.Truth.assertThat;
 import static java.util.concurrent.TimeUnit.MILLISECONDS;
-import static org.robolectric.shadows.ShadowBaseLooper.shadowMainLooper;
+import static org.robolectric.shadows.ShadowLooper.shadowMainLooper;
 
 import androidx.test.core.app.ApplicationProvider;
 import androidx.test.ext.junit.runners.AndroidJUnit4;