Fix for http://b/issue?id=5064532

5064532: GridLayout with initial "stretchy" row and "gone" view in last row doesn't stretch properly

Also:

. Infer stretchibility from whether or not gravity is defined.
. Make algorithms for handling flexibility within cell groups consistent
  with those acting between cells groups (via constraint system).
. Hide and deprecate methods taking flexibility argument.
. Hide and deprecate CAN_STRETCH constant.

Both deprecated features will be removed after references are removed from platform.

Change-Id: Iabf2bf19f35cf30b8ec49c99b49a0550fd495125