Migrate LAYOUT_STABLE in new insets world

WindowInsets.getSystemInsets returns the stable insets if the
window has set SYSTEM_UI_FLAG_STABLE set.

Furthermore, consuming stable will also consume system insets, and
consuming system insets will consume stable. However, DecorView
will not longer consume stable.

Bug: 118118435
Test: InsetsStateTest, WindowInsetsTest
Change-Id: I232e90a6ec34036e830a53437cf32a2b794c5257