threadshare: some src ts-elements reset `configured_caps` when they shouldn't

The following discussion from !328 (merged) should be addressed:

• @fengalin started a discussion: (+1 comment)

  Not a consequence of this MR, but I think there's an issue with caps management. As an example, in AppSrc::stop we call AppSrcPadHandler::reset which sets the configured_caps to None. If we switch back to Playing after Stopped, there will no longer be a configured_caps. I think we should set configured_caps to None in AppSrc::unprepare. Will address this in a separate MR if you confirm.