Use the default top clock margin on h800 devices

On devices that are at least 800px tall, use the default
18dp top margin instead of a greater margin.

We want to make sure we can always see large media
and a conversation group on LS.

Test: play media and post a group notification,
see they fit on the lock screen
Fixes: 214257536

Change-Id: I9a81ed0497002b4b22aa7c1cbe3394411dee2f41