Don't let IME window fit status bar

1. So that the IME position won't be affected by the status bar
   position.
2. When status bar is hidden, the fullscreen IME can fill the status bar
   area.

Since IME can be behind status bar now, we set the status bar color to
be transparent, so that there won't be a visible color view at the
status bar area.

Fix: 168657591
Test: 1. Steps in the bug.
      2. Play WindowInsetsTests with combinations of system bar
         visibilities, navigation mode, screen orientation, and display
         cutout.
      3. Show/hide status bar while IME is visible, and see if there is
         any flicker.
Change-Id: Ia133a62cfa98c47c0d3f8d118947a18246dd8d4f