[Flash] Allow flash.Lib.current to be a flash.display.Sprite

Currently, flash.Lib.current can only be a MovieClip, however, a main class on a flash app can be a Sprite.
This causes issues mainly on Haxe Libraries compiled to a SWC, since a user cannot do haxe.initSwc(this); on their main class unless it extends MovieClip (trying to cast to MovieClip still results in current Beeing null).
This is especially am issue if they the user uses a framework whose main class doesn't extend MovieClip but Sprite.