Rename TaskDescription's statusBarAppearance to systemBarsAppearance

This value is populated from
WindowInsetsController#setSystemBarsAppearance and communicates more
appearance flags than just the status bar, so it should be named
accordingly.

Bug: 325125042
Test: m && atest ActivityManagerTest DesktopModeWindowDecorationTests
Change-Id: Id1b74f707cff12d789ebd1637be7bfb5959c7d5f