Add GRALLOC_FRAMEBUFFER_NUM configuration

Add config to set the NUM_BUFFERS. Set NUM_BUFFERS to 1 can
prevent page flipping and set it to a larger number can allow
userspace to fill up the following frame earlier.

Bug: 205917753
Change-Id: Ibf4a52a1e35f4a2404008a48b637cb82c4b46365
2 files changed