Don't force RGB-565 for tablet status bar

RGBX-8888 is preferred on some devices because the HW composer doesn't
support RGB-565.  SurfaceFlinger can map PixelFormat.OPAQUE to RGB-565
or RGBX-8888 depending on the NO_RGBX_8888 flag.

Change-Id: I6848b11f694188b606a5547b6dd386d933e30601
Signed-off-by: Greg Hackmann <ghackmann@google.com>