x/mobile/app: warnings on compiling

[hajimehoshi]

I saw these warnings when compiling an iOS application with gomobile:

# golang.org/x/mobile/app
darwin_ios.m:89:74: warning: 'statusBarOrientation' is deprecated: first deprecated in iOS 13.0 - Use the interfaceOrientation property of the window scene instead. [-Wdeprecated-declarations]
/Applications/Xcode.app/Contents/Developer/Platforms/iPhoneOS.platform/Developer/SDKs/iPhoneOS15.2.sdk/System/Library/Frameworks/UIKit.framework/Headers/UIApplication.h:146:55: note: property 'statusBarOrientation' is declared deprecated here
/Applications/Xcode.app/Contents/Developer/Platforms/iPhoneOS.platform/Developer/SDKs/iPhoneOS15.2.sdk/System/Library/Frameworks/UIKit.framework/Headers/UIApplication.h:146:55: note: 'statusBarOrientation' has been explicitly marked deprecated here
darwin_ios.m:97:75: warning: 'statusBarOrientation' is deprecated: first deprecated in iOS 13.0 - Use the interfaceOrientation property of the window scene instead. [-Wdeprecated-declarations]
/Applications/Xcode.app/Contents/Developer/Platforms/iPhoneOS.platform/Developer/SDKs/iPhoneOS15.2.sdk/System/Library/Frameworks/UIKit.framework/Headers/UIApplication.h:146:55: note: property 'statusBarOrientation' is declared deprecated here
/Applications/Xcode.app/Contents/Developer/Platforms/iPhoneOS.platform/Developer/SDKs/iPhoneOS15.2.sdk/System/Library/Frameworks/UIKit.framework/Headers/UIApplication.h:146:55: note: 'statusBarOrientation' has been explicitly marked deprecated here
darwin_ios.m:140:77: warning: null passed to a callee that requires a non-null argument [-Wnonnull]

Gomobile version: 4a8be17bd2e3f4793a871839e766cbea7e999464

/CC @hyangah

[caiwenju]

Has the problem been solved at last？