Finish cleaning DataSource implementations.

- Enfroce read returns 0 if readLength==0 everywhere.
- Fixes and simplifications for CronetDataSource.

-------------
Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=135138232

library/src/main/java/com/google/android/exoplayer2/upstream/FileDataSource.java

@@ -80,7 +80,9 @@ public final class FileDataSource implements DataSource {
 
   @Override
   public int read(byte[] buffer, int offset, int readLength) throws FileDataSourceException {
-    if (bytesRemaining == 0) {
+    if (readLength == 0) {
+      return 0;
+    } else if (bytesRemaining == 0) {
       return C.RESULT_END_OF_INPUT;
     } else {
       int bytesRead;

library/src/main/java/com/google/android/exoplayer2/upstream/UdpDataSource.java

@@ -129,6 +129,10 @@ public final class UdpDataSource implements DataSource {
 
   @Override
   public int read(byte[] buffer, int offset, int readLength) throws UdpDataSourceException {
+    if (readLength == 0) {
+      return 0;
+    }
+
     if (packetRemaining == 0) {
       // We've read all of the data from the current packet. Get another.
       try {
@@ -136,7 +140,6 @@ public final class UdpDataSource implements DataSource {
       } catch (IOException e) {
         throw new UdpDataSourceException(e);
       }
-
       packetRemaining = packet.getLength();
       if (listener != null) {
         listener.onBytesTransferred(this, packetRemaining);

library/src/main/java/com/google/android/exoplayer2/upstream/cache/CacheDataSource.java

@@ -194,12 +194,15 @@ public final class CacheDataSource implements DataSource {
   }
 
   @Override
-  public int read(byte[] buffer, int offset, int max) throws IOException {
+  public int read(byte[] buffer, int offset, int readLength) throws IOException {
+    if (readLength == 0) {
+      return 0;
+    }
     if (bytesRemaining == 0) {
       return C.RESULT_END_OF_INPUT;
     }
     try {
-      int bytesRead = currentDataSource.read(buffer, offset, max);
+      int bytesRead = currentDataSource.read(buffer, offset, readLength);
       if (bytesRead >= 0) {
         if (currentDataSource == cacheReadDataSource) {
           totalCachedBytesRead += bytesRead;
@@ -218,7 +221,7 @@ public final class CacheDataSource implements DataSource {
         closeCurrentSource();
         if (bytesRemaining > 0 || bytesRemaining == C.LENGTH_UNSET) {
           if (openNextSource(false)) {
-            return read(buffer, offset, max);
+            return read(buffer, offset, readLength);
           }
         }
       }