Fix issue #20655182: API Review: ViewAssistStructure

Fix the various view assist related APIs.

Also remove the blockAssist view attribute, and instead use
the window's FLAG_SECURE to drive blocking of the entire
hierarchy (which is semantically correct, and will protect
existing apps that have already indicated they need it).

Change-Id: I6beebc86b202809cba0a356cae9607d8d0fb5e78