[ADI][18/N] Add VERIFICATION_USER_ACTION_NEEDED_REASON constants

Previously, the verification failure reason code constants are used in
both the result sent to the installer, as well as the user confirmation
dialogs. This caused confusion and made it difficult to extend the user
confirmation reasons without adding failure reasons. This CL decouples the
two types of reasons, and clarifies their usages in the javadocs.

+ Update javadoc to reflect the different behaviors of installers based
  on their target SDK and privileged status

+ Add constants for verification user action reasons which are now separated
  from the verification failure reasons. The user action reasons will be
exclusively used in the user confirmation dialogs, wherease the
verification failure reasons are used to inform the installer.

+ Refactor the code a bit to make it more concise.

+ Update the value of the failure reason extra in a couple of places to match
  the expected behavior described in the javadoc.

FLAG: android.content.pm.verification_service
BUG: 360129657
Test: cts tests to be added
API-Coverage-Bug: 367776952

Merged-In: I34f8672453e07fdb7b448d6ac44a807fcceca0d6
Change-Id: I34f8672453e07fdb7b448d6ac44a807fcceca0d6