Finish auto UI transactions when app moves to background

[philipphofmann]

Description

Finish auto-generated UIViewController and user interaction transactions when the app moves to the background. The SentryUIViewControllerPerformanceTracker finishes the transaction on viewWillDisappear, but there might be edge cases, as when an alert appears and the app then moves to the background.

sentry-cocoa/Sources/Sentry/SentryUIViewControllerPerformanceTracker.m

Lines 266 to 274 in c6a8035

	- (void)viewControllerViewWillDisappear:(UIViewController *)controller
	callbackToOrigin:(void (^)(void))callbackToOrigin
	{
	[self finishTransaction:controller
	status:kSentrySpanStatusCancelled
	lifecycleMethod:@"viewWillDisappear"
	callbackToOrigin:callbackToOrigin];
	}

The safest approach is to finish all auto-generated UI transactions when the app moves to the background. This came up in an internal discussion around profiling.