Removed Settings.Global.LAST_ACTIVE_USER_ID.

This property was only used by Automotive and was added on frameworks/base
because it used be accessed by a class on frameworks/opt/services/car, but
it's now only accessed under packages/services/car (so it was moved to
CarSettings).

Test: atest SettingsBackupTest
Bug: 156567994

Change-Id: Idaff4a249600517c5250cd3821d75ce85fc9eeee