Fix -Wcast-qual warnings.

Fix -Wcast-qual warnings.

The Android.mk file enables -Wcast-qual and clang finds problems in
gralloc0.cc where the constness of a pointer is being cast away. AOSP
master is setting -Werror, so these warnings break the build. Fix them.

Change-Id: I8820efab34a15d0653b89303d5390fb9f3eee12d
Reviewed-on: https://chromium-review.googlesource.com/1005719
Commit-Ready: Gurchetan Singh <gurchetansingh@chromium.org>
Tested-by: Alistair Strachan <astrachan@google.com>
Reviewed-by: Alistair Strachan <astrachan@google.com>
Reviewed-by: Gurchetan Singh <gurchetansingh@chromium.org>