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_USE_MHEAP_SCREENSHOT board flag
and requires the ro.bq.gpu_to_cpu_unsupported property set to 1
in order to work.