Deprecated UserManager.getUsers(excludeDying) / added getAliveUsers()

The existing method is confusing (the argument used to be called
includeDying) and it puts the burden on the caller (which need to
understand what the parameter means).

Furthermore:

- The majority of calls are for getUsers(excludeDying=true).
- The calls for getUsers(excludeDying=false) are equivalent to
  calls to getUsers()

Test: m
Test: a VpnTest ConnectivityServiceTest PermissionMonitorTest

Bug: 157921703
Change-Id: Ife767a40b7b7790ba28b5377046de822ddbf275c
Merged-In: Ife767a40b7b7790ba28b5377046de822ddbf275c
(cherry picked from commit 6dc6d2b9)