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
opengl: allow direct TLS access for ARMv6-VFP arch

When ARCH_ARM_HAVE_TLS_REGISTER is true, build opengl libraries
in arm mode (to workaround the missing MCR/MRC registers in thumb
mode).

Change-Id: I319e40d333c5b6808ce098164e27df9064bff706
surfaceflinger: disable 16bpp surface option by default

This should be enabled either via a device config or by user preference
using the Settings -> Performance option. Without this change, the
default value in Settings will be incorrect if the property is unset.

Change-Id: Ieeab42c3c7102ba93cae07e2096fceaf5218984b
Revert "opengl: allow direct TLS access for ARMv6-VFP arch"

This reverts commit d807ac055a1fb072ce255c9b3f00476da5e8607a.

Change-Id: I8380749758e2745025f34b8a9425e2af73c53000