Use UserHandle.SYSTEM for ENABLE_ROLLBACK broadcast.

In the case of staged installs, the user associated with the install may
not be running yet, which would cause the broadcast to fail and lead to
deadlock on boot. Use the SYSTEM user instead, which should always be up
and running.

Bug: 129407914
Test: Manually add multiple users to device,
      $ adb install --user 10 --staged --enable-rollback foo.apk
      Wait a few seconds for session ready.
      $ adb reboot
      Device reboots.

Change-Id: I64363651b6b3660b104cf0a53ec371bef6f5bbc3