Recycle userIds after hitting the limit

Previously the system had to be rebooted to allow reusing IDs of removed
users.

Now removed userIds will be recycled after hitting MAX_USER_ID limit.
In this stage all mRemovingUserIds are released, with the exception of ids
stored in an LRU queue - mRecentlyRemovedIds

Added debug flag RELEASE_DELETED_USER_ID to allow reusing of userIds
immediately after the user is deleted.

Bug: 28822373
Change-Id: Ibde562c69efc1533dbca2f1f8d919bee7473644f