fix a position display bug

fix a bug:
When the app paused, and then back , the position of pois is wrong,
add method as follows:
- (void)applicationWillResignActive:(UIApplication *)application {
    [_viewController beforeApplicationResignActive];
}
- (void)beforeApplicationResignActive
{
    ARView *arView = (ARView *)self.view;
    [arView stop];
    [self stopTimer];// 开始定时器
    NSLog(@"beforeApplicationResignActive");
}

- (void)afterApplicationDidBecomeActive
{
    NSLog(@"afterApplicationDidBecomeActive");
    ARView *arView = (ARView *)self.view;
    [arView start];
    [self startTimer];// 开始定时器
}

- (void)applicationDidBecomeActive:(UIApplication *)application {
    [_viewController afterApplicationDidBecomeActive];
}