Made the expand button positioning conditional on expanded state

The expand button now allows content to flow below the header
when expanded and is dynamically placed in the layout
to push over the content.
Also made sure that match parent views don't artificially
increase the size of RemeasurableLinearLayouts as otherwise
the content would be stuck too big at times.

Bug: 150905003
Test: Visual, observe expand buttons when expanded / collapsed
Change-Id: I933b8a74ab6cacd48c98b1a9667b0e30c82bda11