Make Track[Group|GroupArray|Selection] consistent.

- I don't think the length is useful in hashCode; if the length
  is different then Arrays.hashCode should account for that.
- "this." just for consistency across these classes.
-------------
Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=116641142

library/src/main/java/com/google/android/exoplayer/TrackGroup.java

@@ -58,7 +58,7 @@ public final class TrackGroup {
     Assertions.checkState(formats.length > 0);
     this.adaptive = adaptive;
     this.formats = formats;
-    length = formats.length;
+    this.length = formats.length;
   }
 
   /**
@@ -77,7 +77,6 @@ public final class TrackGroup {
       int result = 17;
       result = 31 * result + (adaptive ? 1231 : 1237);
       result = 31 * result + Arrays.hashCode(formats);
-      result = 31 * result + length;
       hashCode = result;
     }
     return hashCode;

library/src/main/java/com/google/android/exoplayer/TrackGroupArray.java

@@ -53,10 +53,7 @@ public final class TrackGroupArray {
   @Override
   public int hashCode() {
     if (hashCode == 0) {
-      int result = 17;
-      result = 31 * result + Arrays.hashCode(trackGroups);
-      result = 31 * result + length;
-      hashCode = result;
+      hashCode = Arrays.hashCode(trackGroups);
     }
     return hashCode;
   }