Remove DecorCaptionView and CAPTION_ON_SHELL flag

The legacy caption on the client side is replaced by the caption in WM
shell. A lot of development has been made on top of the shell caption
that the legacy caption is way too stale from desktop mode's
perspective.

In the process of removing code, I found there is a public API
Window#setDecorCaptionShade that became useless after moving captions to
WM shell. We should consider if rewiring is necessary based on UX input
and the API usage among apps.

The removal of CAPTION_ON_SHELL should result in the flag being on
permanently.

Bug: 333724879
Bug: 328668781
Test: Launch an app in CF PC and the caption looks OK.
Test: atest InsetsControllerTest
Test: WmTests in presubmit tests.
Test: FrameworksCoreTests in presubmit tests.
Test: WM related CTS tests in presubmit tests.
Test: atest WindowDecorationTests
Flag: NA
Change-Id: If8ac1cc9e8a81f63f9960768aea8d2016a67c4f9