Improve Intent behavior of PlayerActivity

[s12f]

I am trying to set mpvKt as external player of Jellyfin, and found some problems:

1. playback tracking:
   1. mptKv failed to handle the "position" in the intent extras(reason: set player.timePos too early, the MPV is not loaded, and reports Property Unavailable Errors)
   2. Jellyfin failed to handle the result "position" in the intent extras(reason: jellyfin need to read the intent action and do some ad-hoc works)
2. external subtitles with movie: mpvKt didn't load the external subtitles

So this PR is trying to solve those problems:

• Delay the setIntentExtras(..) function, and I check the mpv-android implement, it uses the onloadCommands(https://github.com/mpv-android/mpv-android/blob/5dca6eb229055a99b0cec5ddd67fc68c78295150/app/src/main/java/is/xyz/mpv/MPVActivity.kt#L272) to delay the timing, so I delay the setIntentExtras(..) function call after receiving MPV_EVENT_FILE_LOADED event.
• Add the action into the result intent
• load external subtitles if intent extras provides related fields like mpv-android and MX player(ref: http://mpv-android.github.io/mpv-android/intent.html)

In the PR, I tests with jellyfin-android with a new PR(jellyfin/jellyfin-android#1531), it works perfect now.

related Issue: jellyfin/jellyfin-android#1481