Turn EmbedLiteAppThreadParent singleton into EmbedLiteApp's exclusive attribute

* Drop static method EmbedLiteAppThreadParent::GetInstance() as only EmbedLiteApp instance is allowed to create and to use EmbedLiteAppThreadParent singleton;
* Decouple EmbedLiteAppThreadParent from EmbedLiteCompositorParent;
* Drop EmbedLiteAppThreadParent::mParentLoop attribute, because EmbedLiteApp and EmbedLiteAppThreadParent live on the same thread and use the same message loop;
* Turn the static function DeferredDestroyCompositor() into private method EmbedLiteCompositorParent::DeferredDestroyCompositor().

WARNING: I couldn't fully test the last change because EmbedLiteCompositorParent instance never receives RecvStop() message.