Gralloc: add chance to use legacy gralloc.

For some mobile phones, the framebuffer num will be error in jellybean,
it's because of this roundUpToPageSize, then the screen of phone will
be jitterde. Bring legacy usage can fix this, and when you want use it,
set TARGET_USE_LEGACY_GRALLOC := true in your BoardConfig.mk

Change-Id: I979c351fcfb6fca274361d94af5da5a379d5bafb