[video_player] fix Timer Leak

[creativecreatorormaybenot]

Description

Currently, when play is called twice, i.e. play is called again before pause, the _timer leaks, which can create very frequent calls of listener and more importantly will never stop notifying, even when the video is paused.

I would say that this is a pretty severe issue as calling play on a controller can easily happen more than once and should not have side effects.

Tests

I tried to add the following integration test:

testWidgets('cancels timers', (WidgetTester tester) async {
      await _controller.initialize();

      var eventCount = 0;
      final listener = () => eventCount++;
      _controller.addListener(listener);

      // Play for a second, then pause, and then wait a second.
      await _controller.play();
      // Call play twice to test if timers leak.
      await _controller.play();
      await tester.pumpAndSettle(_playDuration);

      final prePauseEventCount = eventCount;
      await _controller.pause();
      await tester.pumpAndSettle(_playDuration);

      // If timers were properly canceled, there should only be one new event
      // (from pausing). If timers were not canceled, there will be more events.
      expect(eventCount, prePauseEventCount + 1);

      _controller.removeListener(listener);
    }, skip: Platform.isIOS);

However, I was not able to get the existing integration tests to succeed, which is why I figured that this can do without a regression test for now.

Checklist

• I read the Contributor Guide and followed the process outlined there for submitting PRs.
• My PR includes unit or integration tests for all changed/updated/fixed behaviors (See Contributor Guide). (more below)
• All existing and new tests are passing.
• I updated/added relevant documentation (doc comments with ///).
• The analyzer (flutter analyze) does not report any problems on my PR.
• I read and followed the Flutter Style Guide.
• The title of the PR starts with the name of the plugin surrounded by square brackets, e.g. [shared_preferences]
• I updated pubspec.yaml with an appropriate new version according to the pub versioning philosophy.
• I updated CHANGELOG.md to add a description of the change.
• I signed the CLA.
• I am willing to follow-up on review comments in a timely manner.

Breaking Change

Does your PR require plugin users to manually update their apps to accommodate your change?

• No, this is not a breaking change.

[creativecreatorormaybenot]

(I have the feeling that the failing smoke test has nothing to do with my PR)

[Ehesp]

LGTM - replicated & confirmed this fix works.

[iskakaushik]

LGTM

[creativecreatorormaybenot]

@Ehesp @iskakaushik Thanks a lot for taking the time ❤

PS will the video_player update be pushed to Pub? I am just wondering as I know it is not automated for flutter/plugins.