Split AdTagLoader out of ImaAdsLoader

In a later change ImaAdsLoader will use multiple AdTagLoaders.

This change shouldn't have any substantial changes in behavior (it's almost
entirely moving code around). An exception is that ImaSdkSettings is configured
when making a request rather than at construction time.

Issue: #3750
PiperOrigin-RevId: 339891712

extensions/ima/src/test/java/com/google/android/exoplayer2/ext/ima/ImaAdsLoaderTest.java

@@ -172,6 +172,7 @@ public final class ImaAdsLoaderTest {
   public void builder_overridesPlayerType() {
     when(mockImaSdkSettings.getPlayerType()).thenReturn("test player type");
     setupPlayback(CONTENT_TIMELINE, PREROLL_CUE_POINTS_SECONDS);
+    imaAdsLoader.start(adsLoaderListener, adViewProvider);
 
     verify(mockImaSdkSettings).setPlayerType("google/exo.ext.ima");
   }