surfaceflinger: allow surface format selection for NO_RGBX_8888 devices

* Before: RGB_565 was chosen as the default surface format
  (which may not be ideal for 32bpp framebuffer devices).
* After: RGB_565 or RGBA_8888 can be chosen, depending on the value
  of the property persist.sys.prefer_16bpp.

Change-Id: I72cd41a50dfe12c522a72108f8ef4d0db445e169