SystemUI: fix empty notification space in shade_locked

In 8e98622a, we started
funneling update notification calls through the handler
to avoid queueing up hundreds of these calls if a
notification became spammy and to only execute one update if many were
queued.

However, some animations and state transitions are expecting the action
to be done immediately after the call. Facilitate these requests by
allowing them to bypass the handler if they want.

Change-Id: Ice14cdaf93a4d370779b4a3e8996d17cd42fdff8
Signed-off-by: Roman Birg <roman@cyngn.com>