docs: Updates to Multi-Window/PiP APIs for Preview 2

Per bug b/27528326 , the minimalSize attribute is removed from N
Preview 2, replaced by height and width-specific attributes.

Per bug b/27942263 , several method names have changed for DP2.

DO NOT SUBMIT until the final pre-DP2 push, this change should not
go live before DP2 releases.

See first comment for doc stage location.

bug: 27927931
bug: 27942263
Change-Id: I011ab244218ab123bd28aa9e8cb3bc8858fff815