Add Builder for ImaAdsLoader and allow early requestAds

Also fix propagation of ad errors that occur when no player
is attached.

Issue: #3548
Issue: #3556

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Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=178767997

RELEASENOTES.md

@@ -51,6 +51,10 @@
   * Fix ad loading when there is no preroll.
   * Fix ad loading when there is no preroll.
   * Add an option to turn off hiding controls during ad playback
   * Add an option to turn off hiding controls during ad playback
     ([#3532](https://github.com/google/ExoPlayer/issues/3532)).
     ([#3532](https://github.com/google/ExoPlayer/issues/3532)).
+  * Support specifying an ads response instead of an ad tag
+    ([#3548](https://github.com/google/ExoPlayer/issues/3548)).
+  * Support overriding the ad load timeout
+    ([#3556](https://github.com/google/ExoPlayer/issues/3556)).
 
 
 ### 2.6.0 ###
 ### 2.6.0 ###