Add unmaximize icon on CaptionWindowDecor

This CL adds an unmaximize icon to differentiate the icon of the
[un]maximize button icon depending on the target windowing mode.

Demo: https://screenshot.googleplex.com/56c2X9M9gNoVSWm

Bug: 358207467
Test: manual
Flag: EXEMPT Dev Tools
Change-Id: Ica1e4c7b163e8ab20d582c95e5fc798942e3f039