Fix "Backup deactivate failed" error

(This change was originally made in aosp/1293893)

Stop the user first before removing it: removeUser() expects all the
user's processes to be stopped first.

Bug: 154933127
Test: run cts -m CtsBackupHostTestCases -t android.cts.backup.MultiUserBackupStateTest#testBackupDeactivatedWhenUserIsRemoved
Change-Id: I0c6c0af2731273de19f0a667de74bd6c976febdc