Fix MediaPeriod position param Javadoc.

prepare and selectTracks receive the position from which any
loading should start, where-as continueLoading receives the
actual playback position. These are different in the case that
a previous period is still being played out.

Also removed "relative to the start of the period" from prepare
documentation because it couldn't really be relative to anything
else.

-------------
Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=172592769

library/core/src/main/java/com/google/android/exoplayer2/source/MediaPeriod.java

@@ -58,8 +58,7 @@ public interface MediaPeriod extends SequenceableLoader {
    *
    *
    * @param callback Callback to receive updates from this period, including being notified when
    * @param callback Callback to receive updates from this period, including being notified when
    *     preparation completes.
    *     preparation completes.
-   * @param positionUs The position in microseconds relative to the start of the period at which to
-   *     start loading data.
+   * @param positionUs The expected starting position, in microseconds.
    */
    */
   void prepare(Callback callback, long positionUs);
   void prepare(Callback callback, long positionUs);
 
 
@@ -103,7 +102,8 @@ public interface MediaPeriod extends SequenceableLoader {
    *     selections.
    *     selections.
    * @param streamResetFlags Will be updated to indicate new sample streams, and sample streams that
    * @param streamResetFlags Will be updated to indicate new sample streams, and sample streams that
    *     have been retained but with the requirement that the consuming renderer be reset.
    *     have been retained but with the requirement that the consuming renderer be reset.
-   * @param positionUs The current playback position in microseconds.
+   * @param positionUs The current playback position in microseconds. If playback of this period has
+    *    not yet started, the value will be the starting position.
    * @return The actual position at which the tracks were enabled, in microseconds.
    * @return The actual position at which the tracks were enabled, in microseconds.
    */
    */
   long selectTracks(TrackSelection[] selections, boolean[] mayRetainStreamFlags,
   long selectTracks(TrackSelection[] selections, boolean[] mayRetainStreamFlags,
@@ -176,7 +176,9 @@ public interface MediaPeriod extends SequenceableLoader {
    * called when the period is permitted to continue loading data. A period may do this both during
    * called when the period is permitted to continue loading data. A period may do this both during
    * and after preparation.
    * and after preparation.
    *
    *
-   * @param positionUs The current playback position.
+   * @param positionUs The current playback position in microseconds. If playback of this period has
+   *     not yet started, the value will be the starting position minus the duration of any media in
+   *     previous periods still to be played.
    * @return True if progress was made, meaning that {@link #getNextLoadPositionUs()} will return
    * @return True if progress was made, meaning that {@link #getNextLoadPositionUs()} will return
    *     a different value than prior to the call. False otherwise.
    *     a different value than prior to the call. False otherwise.
    */
    */

library/core/src/main/java/com/google/android/exoplayer2/source/SequenceableLoader.java

@@ -52,7 +52,9 @@ public interface SequenceableLoader {
   /**
   /**
    * Attempts to continue loading.
    * Attempts to continue loading.
    *
    *
-   * @param positionUs The current playback position.
+   * @param positionUs The current playback position in microseconds. If playback of the period to
+   *     which this loader belongs has not yet started, the value will be the period's starting
+   *     position minus the duration of any media in previous periods still to be played.
    * @return True if progress was made, meaning that {@link #getNextLoadPositionUs()} will return
    * @return True if progress was made, meaning that {@link #getNextLoadPositionUs()} will return
    *     a different value than prior to the call. False otherwise.
    *     a different value than prior to the call. False otherwise.
    */
    */