Refactor #6.0: Add SampleSources for DASH and SS.

This is a mechanical change. The two new SampleSource classes
are forked from MultiSampleSource, with the logic that previously
was in the demo app's SourceBuilder methods copied into the
corresponding constructors. Subsequent steps will:

1. Pull the initial manifest processing from DashChunkSource and
SmoothStreamingChunkSource into the new SampleSource classes.
2. Remove the construction of ChunkSampleSource instances from
the constructors, instead instantiating children only as needed
when tracks are enabled.
3. Simplify ChunkSampleSource down into a ChunkTrackStream object.
-------------
Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=121001021

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

@@ -144,6 +144,32 @@ public final class C {
    */
   public static final int TRACK_TYPE_TEXT = 3;
 
+  /**
+   * A default size in bytes for an individual allocation that forms part of a larger buffer.
+   */
+  public static final int DEFAULT_BUFFER_SEGMENT_SIZE = 64 * 1024;
+
+  /**
+   * A default size in bytes for a video buffer.
+   */
+  public static final int DEFAULT_VIDEO_BUFFER_SIZE = 200 * DEFAULT_BUFFER_SEGMENT_SIZE;
+
+  /**
+   * A default size in bytes for an audio buffer.
+   */
+  public static final int DEFAULT_AUDIO_BUFFER_SIZE = 54 * DEFAULT_BUFFER_SEGMENT_SIZE;
+
+  /**
+   * A default size in bytes for a text buffer.
+   */
+  public static final int DEFAULT_TEXT_BUFFER_SIZE = 2 * DEFAULT_BUFFER_SEGMENT_SIZE;
+
+  /**
+   * A default size in bytes for a muxed buffer (e.g. containing video, audio and text).
+   */
+  public static final int DEFAULT_MUXED_BUFFER_SIZE = DEFAULT_VIDEO_BUFFER_SIZE
+      + DEFAULT_AUDIO_BUFFER_SIZE + DEFAULT_TEXT_BUFFER_SIZE;
+
   private C() {}
 
 }