Fix NPE in app hibernation dump command.

Incorrectly used index instead of the user id to key into the
SparseArray, causing a null value to be returned instead of the actual
states map for the user. This fixes the issue.

Bug: 199935092
Test: Make a new user; adb shell dumpsys app_hibernation
Change-Id: I3f6fcfbaa7a5e83bcf2be46c9e05e5f2f76adf52
(cherry picked from commit 984ea425)