Always distribute excess space in LinearLayout measurement

Previously, excess space was added to existing measured dimensions.
This consistently resulted in incorrect allocation of excess space,
since the delta already included the height of any measured children
rather than just the excess space itself.

This CL ensures that excess space is always distributed according to the
layout weights.

Bug: 22810327
Change-Id: I482a553c469169769cc40ab3d88b4a44023f3eb5