Intercept activity start to check for notification permissions

The dialog is shown if the app is not a signature app, the system is
ready, and the app has created at least one notification channel.
Also applies the SHOULD_SHOW_REQUEST flag to all packages with implicit
POST_NOTIFICATIONS permissions

Bug: 194833441
Test: atest NotificationPermissionTest
Change-Id: I4cb8cc7bcc3635f55e291f176f722dd420a4a1bb