Remove isFirstWindow/isLastWindow from Timeline.

These methods are only used in one place, and offer duplicate
functionality to checking getNext(Previous)WindowIndex == C.INDEX_UNSET.

-------------
Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=165910258

library/core/src/main/java/com/google/android/exoplayer2/Timeline.java

@@ -594,30 +594,6 @@ public abstract class Timeline {
   }
 
   /**
-   * Returns whether the given window is the last window of the timeline depending on the
-   * {@code repeatMode}.
-   *
-   * @param windowIndex A window index.
-   * @param repeatMode A repeat mode.
-   * @return Whether the window of the given index is the last window of the timeline.
-   */
-  public final boolean isLastWindow(int windowIndex, @Player.RepeatMode int repeatMode) {
-    return getNextWindowIndex(windowIndex, repeatMode) == C.INDEX_UNSET;
-  }
-
-  /**
-   * Returns whether the given window is the first window of the timeline depending on the
-   * {@code repeatMode}.
-   *
-   * @param windowIndex A window index.
-   * @param repeatMode A repeat mode.
-   * @return Whether the window of the given index is the first window of the timeline.
-   */
-  public final boolean isFirstWindow(int windowIndex, @Player.RepeatMode int repeatMode) {
-    return getPreviousWindowIndex(windowIndex, repeatMode) == C.INDEX_UNSET;
-  }
-
-  /**
    * Populates a {@link Window} with data for the window at the specified index. Does not populate
    * {@link Window#id}.
    *

library/ui/src/main/java/com/google/android/exoplayer2/ui/PlaybackControlView.java

@@ -675,9 +675,10 @@ public class PlaybackControlView extends FrameLayout {
       int windowIndex = player.getCurrentWindowIndex();
       timeline.getWindow(windowIndex, window);
       isSeekable = window.isSeekable;
-      enablePrevious = !timeline.isFirstWindow(windowIndex, player.getRepeatMode())
-          || isSeekable || !window.isDynamic;
-      enableNext = !timeline.isLastWindow(windowIndex, player.getRepeatMode()) || window.isDynamic;
+      enablePrevious = isSeekable || !window.isDynamic
+          || timeline.getPreviousWindowIndex(windowIndex, player.getRepeatMode()) != C.INDEX_UNSET;
+      enableNext = window.isDynamic
+          || timeline.getNextWindowIndex(windowIndex, player.getRepeatMode()) != C.INDEX_UNSET;
       if (player.isPlayingAd()) {
         // Always hide player controls during ads.
         hide();