Move IWindowContainer to WindowContainer

Since Task/Stack are merged and we're able to implement tiles
atop of Task, the IWindowContainer impl doesn't need to be in
ConfigurationContainer anymore. This removes some casting.

Also moved the Focusable interface to WC since it isn't really
needed for ConfigurationContainers.

Bug: 133381284
Test: tests still pass
Change-Id: I4adc52ca6b4e5da1cf5aaf410f066d7e2a70e6dd