Add an intermediate dimens resource for 540dp device.

A 540dp device causes keyguard to select SlidingChallengeLayout
and a height of 400dp which isn't enough space for the security
area.  This fix adds an intermediate value to grow the security
area slightly.

Fixes bug 11344424

Change-Id: I35c979f619c593b604e4bf45afd3e591329229e4