Lockscreen: allow widgets to use more space on tablets

aka: use that gorgeous screen

PS2: use @dimen/keyguard_widget_pager_max_height so we can use overlays

Change-Id: I0a99f98c868aa88014a3c2596b18f3cbe3480d96
Signed-off-by: Dirk Rettschlag <dirk.rettschlag@gmail.com>