android: add helper to convert buffer_handle_t to gralloc_handle_t ptr

Clients frequently need to convert a buffer_handle_t (aka
native_handle_t *) to a gralloc_handle_t ptr. This is a simple cast, but
add an inline function to do the conversion.

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