Don't force IME drawing system bar backgrounds

If IME doesn't have FLAG_DRAWS_SYSTEM_BAR_BACKGROUNDS, DecorView won't
show the color views for providing background color of system bars.
DecorView will call Window#onDrawLegacyNavigationBarBackgroundChanged
when the flag is changed. This gives NavigationBarController a chance
to decide whether to draw the legacy navigation bar background for the
IME or not.

Bug: 201375975
Test: Show IME in combinations of
      a. floating mode or non-floating mode
      b. gesture navigation or 3-button navigation
      c. taskbar or navigation bar
      d. light theme or dark theme
      And see if there is anything unexpected
Change-Id: If7ac32e2b874c04a260d542cb6a7595288aae182