Slate ephemeral user deletion in systemReady()

After a reboot,
ephemeral users are currently slated for deletion upon
UserManager.onBootPhase(PHASE_ACTIVITY_MANAGER_READY).
However, ephemeral users are already somewhat removed prior
to this, without it being reported, which can cause severe
problems.

Here, we slate ephemeral users for deletion slightly earlier,
in UserManager.systemReady().

Bug: 181291095

Test: atest android.host.multiuser.EphemeralTest#testRebootAndRemoveEphemeralUser_withAccount

Test: manual:
adb shell pm create-user --ephemeral Ephy
switch to the user, connect to the internet, and set up an account
reboot the device (from within that ephemeral user)
observe no boot-loop

Change-Id: I0f60785a1e67945f42d8332ca4d09d06145a3347