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
hax: Split the exynos4/tegra legacy code paths
They hang up at different places, and require different fixes.
Tested on t3 and e4
Change-Id: I46257a7e9e6b6e093a5cb92eaaaef4bdd0435a12