feat(player): move screenshot writing task to background thread

[dhanuarf]

Also added snackbar to notify user.

[FineFindus]

Thanks, please split this into two different commits, as it's two different changes (this makes it easier to follow the history).

Also, I think

• having a snackbar to show that a screenshot is being created isn't really necessary, as that is already obvious when the user is asked to choose a file and generally saving the file is relatively quick
• we should use a toast here, instead of a snackbar, considering the user does not have any useful interaction, similar to how we show a toast when a SponsorBlock segment is skipped. Alternatively I'm also open to adding a share button to the snackbar.

[dhanuarf]

• having a snackbar to show that a screenshot is being created isn't really necessary, as that is already obvious when the user is asked to choose a file and generally saving the file is relatively quick

This depends on the quality and/or the content of what is being captured. PNG is quite costly when it comes to complex/detailed picture. It could take up to a few seconds depending on the device. I think it would be awkward if we only notify the user after it's succesfully saved, because the wait time could be several seconds.

But I'm fine removing it though.

• we should use a toast here, instead of a snackbar, considering the user does not have any useful interaction, similar to how we show a toast when a SponsorBlock segment is skipped.

The reason I use snackbar is just to match the material design. Toast is displayed by the system framework, so different android version have different looks. So if we really follow the material design I think we need start to use snackbar from now on?

Alternatively I'm also open to adding a share button to the snackbar.

I think that's a great idea.