Added WindowContainer.removeImmediately to support immediate removal

There are 2 types of window conatiner removals. The ones that happen
immediately and the ones that are deferred until the system is in
a good state to handle them. WindowContainer and its child classes
now support both through WC.removeImmediately() and WC.removeIfPossible()
methods.
The method names create a consistency in the codebase and also makes is
obvious what the methods will do.

Bug: 30060889
Change-Id: I459c2eef17e20cefc42e9cc542c9a3c69fc9b898