[2/N] WindowDecor refactor: WindowDecoration2 and CaptionController

This change divides the current WindowDecoration class into two new
classes: WindowDecoration2 and CaptionController. Everything related to
the caption is put in the latter while everything else is put in
WindowDecoration2.

Bug: 409648813
Flag: com.android.window.flags.enable_window_decoration_refactor
Test: m
Change-Id: Icc8d987e0e81a2478dcd9b50620c93d675e53256