Make screen block view first responder.

Signal/src/util/OWSScreenLockUI.m

@@ -438,6 +438,8 @@ const UIWindowLevel UIWindowLevel_Background = -1.f;
         // Show the blocking window in front of the status bar.
         self.screenBlockingWindow.windowLevel = UIWindowLevelStatusBar + 1;
         self.rootWindow.hidden = YES;
+        [self.screenBlockingWindow becomeFirstResponder];
+        OWSAssert([self.screenBlockingWindow isFirstResponder]);
     } else {
         self.screenBlockingWindow.windowLevel = UIWindowLevel_Background;
         [self.rootWindow makeKeyAndVisible];

SignalMessaging/ViewControllers/ScreenLockViewController.m

@@ -136,4 +136,9 @@ NSString *NSStringForScreenLockUIState(ScreenLockUIState value)
     [self.delegate unlockButtonWasTapped];
 }
 
+- (BOOL)canBecomeFirstResponder
+{
+    return YES;
+}
+
 @end