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, Exynos4 abd QCOM 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.

Original link: http://review.cyanogenmod.org/#/c/53162/

Conflicts:

	libs/gui/Android.mk

Change-Id: I48d44c6ac4cdcb16d835fe163a27de57930b9a38