Show user switch message for non-system user

So far, a DO could only be on system user, and the DO is the only one
who can set user switch messages. With HSUM, this changes, as the DO can
also live in user 10. Change the API and calls to allow for user switch
from and to messages to be configued with a given user, and not a
hardcoded system user.

Fix: 391605774
Test: Manual on tangorpro_hsum:
  m
  adevice update
  g4d -f testdpc
  bash third_party/java_src/testdpc/build-and-install.sh
  adb shell dpm set-device-owner --user 10 com.afwsamples.testdpc/com.afwsamples.testdpc.DeviceAdminReceiver
  Configure user switching message on the device, add a user and switch
Test: CTS verifier user switch message test
Test: btest UserSwitchingMessageTest -n
Flag: EXEMPT BUG_FIX
Change-Id: Ife3cd72690df1172cb334958f5d3f392d425162f