PHPhotoLibrary: Register change observer after asking for permission

[selanthiraiyan]

Fixes #375

Description

As proposed in https://developer.apple.com/forums/thread/696804 this PR register's PHPhotoLibrary change observer after asking for permission from the user.

Steps to test

• Using Xcode 13.2.1 - Run the example app in a device/simulator running iOS 15.2 and make sure that you can access photos without any issues.
• Also, make sure that the changes done to the photos library reflect in the app. (To ensure that the observer is working as expected)

Note: I have only tested the above points in a simulator and it would be great if you can test this in a device.

[SergioEstevao]

@jkmassel I've tested this fix and it solves a critical crash on WPMediaPicker for iOS version 15.2

After this PR is merged we should create a new version of the pod and update all the projects that use this pod.

From what I see the issue identified only happens when the apps are installed for the first and it request permissions to access the device Media library. So this may not affect existing users, but can/will affect new users of the apps.