Fix Seeker.getTimeUs for positions before the first frame.

Issue: #1038
-------------
Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=109906628

library/src/main/java/com/google/android/exoplayer/extractor/mp3/ConstantBitrateSeeker.java

@@ -47,7 +47,8 @@ import com.google.android.exoplayer.C;
 
   @Override
   public long getTimeUs(long position) {
-    return ((position - firstFramePosition) * C.MICROS_PER_SECOND * BITS_PER_BYTE) / bitrate;
+    return (Math.max(0, position - firstFramePosition) * C.MICROS_PER_SECOND * BITS_PER_BYTE)
+        / bitrate;
   }
 
   @Override

library/src/main/java/com/google/android/exoplayer/extractor/mp3/XingSeeker.java

@@ -130,7 +130,7 @@ import com.google.android.exoplayer.util.Util;
 
   @Override
   public long getTimeUs(long position) {
-    if (!isSeekable()) {
+    if (!isSeekable() || position < firstFramePosition) {
       return 0L;
     }
     double offsetByte = 256.0 * (position - firstFramePosition) / sizeBytes;