Remove unused WINDOW_DECOR

WINDOW_DECOR has been defined since the new insets framework of API 30,
but it is never used. This CL removes it.

This CL also removes the ViewDebug annotation of mFitInsetsTypes and
mFitInsetsSides. The string conversion can be done by
WindowInsets.Type.toString and WindowInsets.Side.toString instead. The
conversion doesn't have to be maintained at 2 places (WindowInsets and
WindowManager) anymore.

Bug: 118118435
Flag: EXEMPT clean up
Test: presubmit
Change-Id: I26dff5c9757571c765be41b69170b324d14700ab