Optimize layout rendering in layoutlib [DO NOT MERGE]

The height of a layout should be zero if it is assigned a layout_weight.
This way, the layout is measured only once and prevents spurious errors.

Bug: https://code.google.com/p/android/issues/detail?id=58398
Change-Id: If49a7480e5eb82cb86780e00f2f5b65ee053fc2a
(cherry-picked from 9e5e993c)