Remove window decor for constructor

The window decor config should be created based on the type of caption
that will be drawn (handle vs header) because the density in the config
can be different for each type. Instead of passing in a config in the
ctor, always calculate it on relayout(), which is when the caption type
is known for the first time. The config isn't needed before relayout().

Bug: 332414819
Test: atest WMShellUnitTests
Change-Id: I9af8a483e648b9b94cddf7037a86bd27f595ed57