Adding conservative @Nullable annotations to Exoplayer v2 Cache + CacheSpan

-------------
Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=165715928

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

@@ -15,6 +15,7 @@
  */
 package com.google.android.exoplayer2.upstream.cache;
 
+import android.support.annotation.Nullable;
 import java.io.File;
 import java.io.IOException;
 import java.util.NavigableSet;
@@ -104,7 +105,7 @@ public interface Cache {
    * @param key The key for which spans should be returned.
    * @return The spans for the key. May be null if there are no such spans.
    */
-  NavigableSet<CacheSpan> getCachedSpans(String key);
+  @Nullable NavigableSet<CacheSpan> getCachedSpans(String key);
 
   /**
    * Returns all keys in the cache.
@@ -151,7 +152,7 @@ public interface Cache {
    * @param position The position of the data being requested.
    * @return The {@link CacheSpan}. Or null if the cache entry is locked.
    */
-  CacheSpan startReadWriteNonBlocking(String key, long position) throws CacheException;
+  @Nullable CacheSpan startReadWriteNonBlocking(String key, long position) throws CacheException;
 
   /**
    * Obtains a cache file into which data can be written. Must only be called when holding a

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

@@ -16,6 +16,7 @@
 package com.google.android.exoplayer2.upstream.cache;
 
 import android.support.annotation.NonNull;
+import android.support.annotation.Nullable;
 import com.google.android.exoplayer2.C;
 import java.io.File;
 
@@ -43,7 +44,7 @@ public class CacheSpan implements Comparable<CacheSpan> {
   /**
    * The file corresponding to this {@link CacheSpan}, or null if {@link #isCached} is false.
    */
-  public final File file;
+  public final @Nullable File file;
   /**
    * The last access timestamp, or {@link C#TIME_UNSET} if {@link #isCached} is false.
    */
@@ -68,11 +69,12 @@ public class CacheSpan implements Comparable<CacheSpan> {
    * @param position The position of the {@link CacheSpan} in the original stream.
    * @param length The length of the {@link CacheSpan}, or {@link C#LENGTH_UNSET} if this is an
    *     open-ended hole.
-   * @param lastAccessTimestamp The last access timestamp, or {@link C#TIME_UNSET} if
-   *     {@link #isCached} is false.
+   * @param lastAccessTimestamp The last access timestamp, or {@link C#TIME_UNSET} if {@link
+   *     #isCached} is false.
    * @param file The file corresponding to this {@link CacheSpan}, or null if it's a hole.
    */
-  public CacheSpan(String key, long position, long length, long lastAccessTimestamp, File file) {
+  public CacheSpan(
+      String key, long position, long length, long lastAccessTimestamp, @Nullable File file) {
     this.key = key;
     this.position = position;
     this.length = length;