Fixed SSM.switchUser() so it doesn't crash the system.

SystemServiceManager (SSM) keeps a cache of the TargetUsers it uses
on user lifecycle events, and this cache is only valid from user
start to user stopped. But on user switch, the callback also needs
the TargetUser for the previous user, so if that user is removed,
currently it would crash.

Fixes: 169027200
Test: adb shell pm create-user ZedsDeadBaby && \
  adb shell am switch-user 10 && sleep 10s && \
  adb shell am switch-user 0 && adb shell pm remove-user 10

Change-Id: Ia5a3ac88040a89a9af5f99502846bb05b7ce680b