android: fix mis-named alloc_handle_t

Fix a typo where alloc_handle_t should be gralloc_handle_t. One still
remains in gralloc_handle_create, but a subsequent commit will fix that
along with other problems in gralloc_handle_create.

Reviewed-by: Robert Foss <robert.foss@collabora.com>
Signed-off-by: Rob Herring <robh@kernel.org>