gralloc: Use ION fd in FB handle

- After mmap, use the meta data ioctl to get the ION fd
  for the framebuffer memory as GPU expects buffers to
  from ION
- Set appropriate flags for the framebuffer private handle
- Remove unused members

Change-Id: I45c4917d9811cd3b3800adf222b92f41f97430c0

libgralloc/gralloc.cpp

@@ -91,7 +91,6 @@ struct private_module_t HAL_MODULE_INFO_SYM = {
     numBuffers: 0,
     bufferMask: 0,
     lock: PTHREAD_MUTEX_INITIALIZER,
-    currentBuffer: 0,
 };
 
 // Open Gralloc device