Merge MP3 sniffing/synchronization functionality.

As part of this change, Extractor.sniff may read/skip (not just peek) if it
returns true. This allows Extractors to avoid parsing the input a second time in
read.
-------------
Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=112683272

library/src/main/java/com/google/android/exoplayer/extractor/Extractor.java

@@ -52,7 +52,12 @@ public interface Extractor {
   /**
    * Returns whether this extractor can extract samples from the {@link ExtractorInput}, which must
    * provide data from the start of the stream.
+   * <p>
+   * If {@code true} is returned, the {@code input}'s reading position may have been modified.
+   * Otherwise, only its peek position may have been modified.
    *
+   * @param input The {@link ExtractorInput} from which data should be peeked/read.
+   * @return Whether this extractor can read the provided input.
    * @throws IOException If an error occurred reading from the input.
    * @throws InterruptedException If the thread was interrupted.
    */