[video] Fix watched state / last played not preserved on internet update

[ksooo]

… of movies / TV show episodes.

Fixes a regression introduced with #20842 (@DaVukovic fyi), Changing the default value of advanced settings m_bVideoLibraryImportWatchedState to true has an unexpected side effect.

When refreshing a movie/episode data from the internet and abovementioned setting is set to true, this triggers a bug that resets the play count of the movie/episode to zero, which means movie/episode is unwatched. Internet update does never provide updated play counts, but the code that stores the new values into the video database ignores this fact.

To reproduce:

1. Ensure importwatchedstate is not manually set to false in advancedsettings.xml
2. Open the Video Info Dialog for a TV show that has some watched episodes
3. Push the Refresh button
4. Confirm to update info including info for all episodes
   ==> All formerly episodes marked watched are now unwatched again.

@enen92 fix is trivial - like we already do for resume points, we need to check validity of the play count before overwriting existing database value. Code review should be easy.

@fuzzard this should be backported.