Simplify demo app by moving EventLogger into core

It seems good to have EventLogger available from the library.
In particular because when app developers use it and then
submit bug reports, it makes it much easier to work out what
happened. It will also allow EventLogger to be used across
our (now multiple) demo apps.

-------------
Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=182389407

RELEASENOTES.md

@@ -69,6 +69,7 @@
   cache after deciding to bypass cache.
 * IMA extension: Add support for playing non-Extractor content MediaSources in
   the IMA demo app ([#3676](https://github.com/google/ExoPlayer/issues/3676)).
+* `EventLogger` moved from the demo app into the core library.
 
 ### 2.6.1 ###
 

demos/main/src/main/java/com/google/android/exoplayer2/demo/PlayerActivity.java

@@ -73,6 +73,7 @@ import com.google.android.exoplayer2.ui.PlayerView;
 import com.google.android.exoplayer2.upstream.DataSource;
 import com.google.android.exoplayer2.upstream.DefaultBandwidthMeter;
 import com.google.android.exoplayer2.upstream.HttpDataSource;
+import com.google.android.exoplayer2.util.EventLogger;
 import com.google.android.exoplayer2.util.Util;
 import java.net.CookieHandler;
 import java.net.CookieManager;

demos/main/src/main/java/com/google/android/exoplayer2/demo/EventLogger.java → library/core/src/main/java/com/google/android/exoplayer2/util/EventLogger.java

@@ -13,7 +13,7 @@
  * See the License for the specific language governing permissions and
  * limitations under the License.
  */
-package com.google.android.exoplayer2.demo;
+package com.google.android.exoplayer2.util;
 
 import android.os.SystemClock;
 import android.util.Log;
@@ -53,8 +53,8 @@ import java.io.IOException;
 import java.text.NumberFormat;
 import java.util.Locale;
 
-/** Logs player events using {@link Log}. */
-/* package */ final class EventLogger
+/** Logs events from {@link Player} and other core components using {@link Log}. */
+public class EventLogger
     implements Player.EventListener,
         MetadataOutput,
         AudioRendererEventListener,