Add cache initializtion signal to CacheEvictor

-------------
Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=124374161

library/src/main/java/com/google/android/exoplayer/upstream/cache/CacheEvictor.java

@@ -21,6 +21,9 @@ package com.google.android.exoplayer.upstream.cache;
  */
 public interface CacheEvictor extends Cache.Listener {
 
+  /** Invoked when cache has beeen initialized. */
+  void onCacheInitialized();
+
   /**
    * Invoked when a writer starts writing to the cache.
    *

library/src/main/java/com/google/android/exoplayer/upstream/cache/LeastRecentlyUsedCacheEvictor.java

@@ -33,6 +33,11 @@ public final class LeastRecentlyUsedCacheEvictor implements CacheEvictor, Compar
     this.leastRecentlyUsed = new TreeSet<>(this);
   }
 
+  @Override
+  public void onCacheInitialized() {
+    // Do nothing.
+  }
+
   @Override
   public void onStartFile(Cache cache, String key, long position, long length) {
     evictCache(cache, length);

library/src/main/java/com/google/android/exoplayer/upstream/cache/NoOpCacheEvictor.java

@@ -24,6 +24,11 @@ package com.google.android.exoplayer.upstream.cache;
  */
 public final class NoOpCacheEvictor implements CacheEvictor {
 
+  @Override
+  public void onCacheInitialized() {
+    // Do nothing.
+  }
+
   @Override
   public void onStartFile(Cache cache, String key, long position, long length) {
     // Do nothing.

library/src/main/java/com/google/android/exoplayer/upstream/cache/SimpleCache.java

@@ -253,6 +253,7 @@ public final class SimpleCache implements Cache {
         }
       }
     }
+    evictor.onCacheInitialized();
   }
 
   /**