separate mHeap-based screenshot from BOARD_EGL_NEEDS_LEGACY_FB

No need for the Exynos/Tegra legacy fb implementation on legacy
qcom. Add a separate flag for mHeap-based screenshot in case of
future extensions. Enable mHeap-based screenshot for legacy qcom
dislay.

Change-Id: I35ff015555f19a5ba2857ad3d765d02eb6b9acde