Added WindowContainer and WindowContainerTest classes.

The WindowContainer class defines common functionality for objects that
can hold windows directly or through their children
(e.g. WindowState, Task, AppWindowToken, ...). The WindowContainerTest
class as the name implies contains tests for the WindowContainer class.

Bug: 30060889
Change-Id: I1dd2c460c56cfc82aee67c033032a96843ed051d
2 files changed