closeWebView fixes for url_launcher

[timtraversy]

Fixing some unnoticed issues from #924. Moved the deallocation of the currentSession object to a closure that is called when the Safari VC is dismissed. Now it is deallocated if the closeWebView function is called, or if the user manually dismissed the VC.

I didn't notice any Android issues while testing. I think sometimes the simulator can take a while to launch a Web View and if it takes longer than 5 seconds it appears like there is a bug, but it is working fine for me.

[KingIdee]

Please, when can we expect this to be merged. Please before the holidays... 🙏

[timtraversy]

Sorry, had some problems rebasing this, not sure why it pinged everybody for review.
Close web view is still broken on iOS.

  [self.viewController presentViewController:self.currentSession.safari
	                                                 animated:YES
	                                              completion:^void() {
	                                                 weakSelf.currentSession = nil;
	                                              }];

This completion handles is called the moment the moment the VS finishes loading, so once the closeWebViewWithResult is called [self.currentSession close] does nothing because currentSession has already been set to nil.

To prevent a memory leak by never setting it to nil, I've added a closure to the FLTUrlLaunchSession so the VS is deallocated if the closeWebView function is called, or if the user manually dismisses the VC.

Other options for handling this: #1145, #1010

[cyanglaz]

Need to add CHANGLOG and pubspec updates.