Prevent launching credential input if managed user is alive

If the user very quickly disables and reenables quietmode the user has
not finished dying and the input for the credential screen can
disappear.
We check if the managed user is still alive, and ignore calls to
showConfirmCredentialToDisableQuietMode() if it is

Change-Id: I0e808bde3c57fa9123783590c1328b74440f703f
Test: atest android.devicepolicy.cts.QuietModeTest
Bug: 303722527
Bug: 293441361