Update IMA documentation for AdsLoader

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Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=171533782

extensions/ima/README.md

 # ExoPlayer IMA extension #
 # ExoPlayer IMA extension #
 
 
-The IMA extension is a [MediaSource][] implementation wrapping the
+The IMA extension is an [AdsLoader][] implementation wrapping the
 [Interactive Media Ads SDK for Android][IMA]. You can use it to insert ads
 [Interactive Media Ads SDK for Android][IMA]. You can use it to insert ads
 alongside content.
 alongside content.
 
 
 [IMA]: https://developers.google.com/interactive-media-ads/docs/sdks/android/
 [IMA]: https://developers.google.com/interactive-media-ads/docs/sdks/android/
-[MediaSource]: https://google.github.io/ExoPlayer/doc/reference/index.html?com/google/android/exoplayer2/source/MediaSource.html
+[AdsLoader]: https://google.github.io/ExoPlayer/doc/reference/index.html?com/google/android/exoplayer2/source/ads/AdsLoader.html
 
 
 ## Getting the extension ##
 ## Getting the extension ##
 
 
@@ -27,7 +27,7 @@ locally. Instructions for doing this can be found in ExoPlayer's
 ## Using the extension ##
 ## Using the extension ##
 
 
 To play ads alongside a single-window content `MediaSource`, prepare the player
 To play ads alongside a single-window content `MediaSource`, prepare the player
-with an `ImaAdsMediaSource` constructed using an `ImaAdsLoader`, the content
+with an `AdsMediaSource` constructed using an `ImaAdsLoader`, the content
 `MediaSource` and an overlay `ViewGroup` on top of the player. Pass an ad tag
 `MediaSource` and an overlay `ViewGroup` on top of the player. Pass an ad tag
 URI from your ad campaign when creating the `ImaAdsLoader`. The IMA
 URI from your ad campaign when creating the `ImaAdsLoader`. The IMA
 documentation includes some [sample ad tags][] for testing.
 documentation includes some [sample ad tags][] for testing.
@@ -38,7 +38,7 @@ background, and are recreated when the player returns to the foreground. When
 playing ads it is necessary to persist ad playback state while in the background
 playing ads it is necessary to persist ad playback state while in the background
 by keeping a reference to the `ImaAdsLoader`. Reuse it when resuming playback of
 by keeping a reference to the `ImaAdsLoader`. Reuse it when resuming playback of
 the same content/ads by passing it in when constructing the new
 the same content/ads by passing it in when constructing the new
-`ImaAdsMediaSource`. It is also important to persist the player position when
+`AdsMediaSource`. It is also important to persist the player position when
 entering the background by storing the value of `player.getContentPosition()`.
 entering the background by storing the value of `player.getContentPosition()`.
 On returning to the foreground, seek to that position before preparing the new
 On returning to the foreground, seek to that position before preparing the new
 player instance. Finally, it is important to call `ImaAdsLoader.release()` when
 player instance. Finally, it is important to call `ImaAdsLoader.release()` when