libgui: Bring back support for mHeap-based screenshots

Older graphics libraries throw a hissy fit when trying to lock
buffers for Surface-based screenshots, on at least Tegra2/3 and
Exynos4 hardware.

This patch depends on the BOARD_EGL_NEEDS_LEGACY_FB board flag
and requires the ro.bq.gpu_to_cpu_unsupported property set to 1
in order to work

Change-Id: I7db955e2cdd120018e349c14290e975788e70ed3

Conflicts:
	libs/gui/Android.mk
	libs/gui/ISurfaceComposer.cpp
	services/surfaceflinger/SurfaceFlinger.cpp
	services/surfaceflinger/SurfaceFlinger.h

	modified:   include/gui/ISurfaceComposer.h
	modified:   include/gui/SurfaceComposerClient.h
	modified:   libs/gui/Android.mk
	modified:   libs/gui/ISurfaceComposer.cpp
	modified:   libs/gui/SurfaceComposerClient.cpp
	modified:   services/surfaceflinger/SurfaceFlinger.cpp
	modified:   services/surfaceflinger/SurfaceFlinger.h