Add a debug command to poll IMMS.Lifecycle#onSwitchUser()

This is a quick workaround for a known limitation that

  adb shell am switch-user <user id>

doesn't wait until com.android.server.SystemService#onSwitchUser()
gets called.

The problem is that UserController.switchUser() immediately returns
soon after enqueuing a task to call back SystemService#onSwitchUser().
This means that on a slow device / environment, there is a chance that
IMMS continues behaving as if a previous user was still interacting
after switching a user.

Ideally there should be a way to wait until all system servers finish
handling SystemService#onSwitchUser() in am shell commands, but it may
require non-trivial amount of changes in ActivityManagerShellCommand.
In order to unblock on-going per-profile IME effort, this CL exposes
an internal state of InputMethodManagerService (IMMS) so that CTS
tests can wait until IMMS.Lifecycle#onSwitchUser() gets called with an
expected user ID as a quick workaround.

Bug: 120784635
Bug: 122469833
Test: adb shell cmd input_method get-last-switch-user-id
Test: atest CtsInputMethodTestCases CtsInputMethodServiceHostTestCases
Change-Id: I07a0120d44bcb3e3d60d51c510e7e566ce424148