Update swizzling trigger

[congsun]

Issue
If Visualizer.sharedInstance is not invoked before app is active, calling start function won't work. This is because the swizzling relies on the appBecomeActive notification, and the notification registration is upon initialization of Visualizer.

Fix

• Put the swizzling function in the initialization process of Visualizer.sharedInstance. This guarantees whenever the start is called, we'll always get the function swizzled.
• Remove the unnecessary bool check: As the singleton is guaranteed to be executed once in the application lifecycle, we won't need to worry about the function gets swizzled back.