Post links opened through the app should include the Reader in the backstack

Expected behavior

Opening a post link through the app should include the main activity in the backstack, so that when we press back we go back to the Reader.

Actual behavior

Pressing back after opening a post link through the app send us back to where we first opened the link.

Steps to reproduce the behavior

1. Send yourself an email with a post link (check below for example of supported URLs).
2. On a mobile device with the app installed, click the link in your email via your email app and choose to open it with the WordPress app.
3. Notice the post in the WordPress app.
4. Press back.
5. Notice that you're back at your email app.

Currently supported URL patterns for post links:

• http[s]://wordpress.com/read/blogs/{blogId}/posts/{postId}
• http[s]://wordpress.com/read/feeds/{feedId}/posts/{feedItemId}
• http[s]://{username}.wordpress.com/{year}/{month}/{day}/{postSlug}

Post link examples:

• https://discover.wordpress.com/2018/03/22/how-to-create-website-logo/
• https://wordpress.com/read/blogs/53424024/posts/32898

Tested on Pixel 3XL, Android 11, WPAndroid 17.1

Start by tracing the app's post-link handling and the main activity entry point for the supported URL patterns. Reproduce the flow from an external email app on Android and inspect the back navigation. Done means pressing back from the opened post returns to the Reader rather than the app that supplied the link.