Collapse Notification panel after tapping CLEAR ALL button

Usually, notification panel is automatically collapsed
after tapping "CLEAR ALL" button but is not collapsed
if all dismissable notifications are scrolled out.

In this case, dismissed notification is still displayed
until notification panel is once collapsed.
And that "dismissed" notification causes several issues.

To avoid this, collapse notification panel automatically
after tapping "CLEAR ALL" button even when all dismissable
notifications are scrolled out.

Fixes: 67246876
Test: manual - post ongoing and dismissable notifications,
               and tap CLEAR ALL button.

Change-Id: I048255cde6fdb4cedd2c3472e345ff63c744af68