feat: add isSeeking to onPlaybackStateChanged

[KrzysztofMoch]

Summary

Add isSeeking to onPlaybackStateChanged callback

Motivation

This is pretty useful for devs that are using custom controls, bcs they can check if onPlaybackStateChanged was made bcs of pause/resume or seeking

fixes #3895

Changes

• add isSeeking to onPlaybackStateChanged
• on android i have created new variable to track seeking (not sure if it's ok but it's working)

Test plan

Tested on android & iOS simulators

[YangJonghun]

Personally, I'd like to be able to track the state of the player rather than individual variables.
Attach expo-video docs link(code link) as a reference.

[freeboub]

this test should be before eventEmitter.playbackStateChanged ?
becuase you are changing isSeeking, but not sending it to app ?! 🤔

[KrzysztofMoch]

Yes that's correct the flow is:

1. when seek "is detected" onPositionDiscontinuity is called and in it we are setting isSeeking to true
2. after seeking is done (and we want to triger onSeek on JS side) two thing can happen:
   A) If player was not paused when user seek -> onIsPlayingChanged will be called
   B) If player was paused when user seek -> we will catch that loading is finished in onBuffering

We are sending event with isSeeking=true bcs we want to inform that onPlaybackStateChanged was called bcs of seeking, and then we will set in back to false

This is needed to check if we should show spiner when user is seeking (custom controls) - otherwise we don't know if onPlaybackStateChanged was called bcs of "pause/resume" or "seeking"

So I believe this is correct

[KrzysztofMoch]

Personally, I'd like to be able to track the state of the player rather than individual variables. Attach expo-video docs link(code link) as a reference.

@YangJonghun Good idea! But let's merge this one - then I will do it in next PR (as PR with this change will deprecate current API (isPlaying, isSeeking)

[freeboub]

@KrzysztofMoch this one conflict

[KrzysztofMoch]

@freeboub Done!

[zoriya]

@KrzysztofMoch I agree with @YangJonghun there, having the state would be better. I'd like to use onPlaybackStateChanged for external changes to play/pause (hardware button, audio focus...) to update the UI.

Currently video buffer or seeking triggers this event. Seeking can be ignored reading the isSeeking value introduced in this PR but buffering need special control since the onBuffer event is called AFTER the onPlaybackStateChanged