Avoid possibility of leaking an activity/service context

The bug here was that we'd create a VideoFrameReleaseTimeHelper
using whatever context DefaultRenderersFactory has, and it would
then hold a reference to that context via DisplayManager. A leak
could then occur if the player outlived the life of the context
used to create it (which would be strange/unusual, but not
impossible).

Issue: #4249

-------------
Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=198747599

library/core/src/main/java/com/google/android/exoplayer2/video/MediaCodecVideoRenderer.java

@@ -205,7 +205,7 @@ public MediaCodecVideoRenderer(Context context, MediaCodecSelector mediaCodecSel
     this.allowedJoiningTimeMs = allowedJoiningTimeMs;
     this.maxDroppedFramesToNotify = maxDroppedFramesToNotify;
     this.context = context.getApplicationContext();
-    frameReleaseTimeHelper = new VideoFrameReleaseTimeHelper(context);
+    frameReleaseTimeHelper = new VideoFrameReleaseTimeHelper(this.context);
     eventDispatcher = new EventDispatcher(eventHandler, eventListener);
     deviceNeedsAutoFrcWorkaround = deviceNeedsAutoFrcWorkaround();
     pendingOutputStreamOffsetsUs = new long[MAX_PENDING_OUTPUT_STREAM_OFFSET_COUNT];

library/core/src/main/java/com/google/android/exoplayer2/video/VideoFrameReleaseTimeHelper.java

@@ -72,8 +72,12 @@ public VideoFrameReleaseTimeHelper() {
    * @param context A context from which information about the default display can be retrieved.
    */
   public VideoFrameReleaseTimeHelper(@Nullable Context context) {
-    windowManager = context == null ? null
-        : (WindowManager) context.getSystemService(Context.WINDOW_SERVICE);
+    if (context != null) {
+      context = context.getApplicationContext();
+      windowManager = (WindowManager) context.getSystemService(Context.WINDOW_SERVICE);
+    } else {
+      windowManager = null;
+    }
     if (windowManager != null) {
       displayListener = Util.SDK_INT >= 17 ? maybeBuildDefaultDisplayListenerV17(context) : null;
       vsyncSampler = VSyncSampler.getInstance();