[video_player] Improve macOS frame management (#5078)

Fixes some logic around getting new frames on macOS:
- The logic to copy a frame had a race condition; the macOS code checks that a frame is available for the current playback time before informing the engine that a frame is available (it's not clear why the iOS code doesn't do this; apparently the engine tolerates returning NULL frames on iOS?), but failed to account for the fact that the current playback time when the (async) request from the engine comes in can be different. This fixes it to remember the last time that was known to be available, and uses that when the current time isn't available. This fixes flickering on playback (since returning NULL on macOS causes the video to vanish until a new frame is available).
- Fixes seek to temporarily restart the display link if the video is paused, rather than telling the engine that a frame is available, because it might not be. This is changed for both macOS and iOS since I don't see any reason this bug couldn't affect iOS as well (although in practice I'm only aware of it being reproducible on macOS).

This extracts the display link code for macOS and iOS into an abstraction, eliminating most of the ifdefing, in order to support the latter (since more code needs to be able to play/pause the display link), which also resolves a TODO from the initial implementation.

There is also some non-trivial addition of factory injection in order to make the code more testable. This is definitely not complete, but it incrementally moves the code toward being more testable than it was before, and allows for testing the display link behavior.

Lastly, this moves some code used by tests to the existing `_Test.h` header, removing redeclarations from unit test files, since we already have a test header and that's our preferred approach for accessing private details in ObjC tests. (Longer term the multi-class mega-file should be broken up more to reduce the need for that.)

Fixes flutter/flutter#136027
Improves flutter/flutter#135999

packages/video_player/video_player_avfoundation/CHANGELOG.md

@@ -1,3 +1,7 @@
+## 2.5.2
+
+* Fixes flickering and seek-while-paused on macOS.
+
 ## 2.5.1
 
 * Updates to  Pigeon 13.

packages/video_player/video_player_avfoundation/darwin/Classes/FVPDisplayLink.h

@@ -0,0 +1,34 @@
+// Copyright 2013 The Flutter Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style license that can be
+// found in the LICENSE file.
+
+#import <Foundation/Foundation.h>
+
+#if TARGET_OS_OSX
+#import <FlutterMacOS/FlutterMacOS.h>
+#else
+#import <Flutter/Flutter.h>
+#endif
+
+// A cross-platform display link abstraction.
+@interface FVPDisplayLink : NSObject
+
+/**
+ * Whether the display link is currently running (i.e., firing events).
+ *
+ * Defaults to NO.
+ */
+@property(nonatomic, assign) BOOL running;
+
+/**
+ * Initializes a display link that calls the given callback when fired.
+ *
+ * The display link starts paused, so must be started, by setting 'running' to YES, before the
+ * callback will fire.
+ */
+- (instancetype)initWithRegistrar:(id<FlutterPluginRegistrar>)registrar
+                         callback:(void (^)(void))callback NS_DESIGNATED_INITIALIZER;
+
+- (instancetype)init NS_UNAVAILABLE;
+
+@end