Fix new notification showing timestamp "now" after turning off DND

DateTimeView won't update timestamp until the view is attached to
window and received TIME_TICK intent.

Update timestamp on onAttachedToWindow().

Test: manual 1) turn on DND 2) send a notification and wait some time 3)
turn off DND and check the timestamp
Fixes: 77970557

Change-Id: Ia8420aacf5b91b0bb9cbec561629ddbfc8de4f67