AssetManager cookies should be int32_t and not void*.

Cookies are really indices into vectors and arrays, so
they don't need to be void*. We choose int32_t instead
of size_t to allow their width to be well specified.

(cherry picked from commit ebfdd0f467e39c3af8d92cade78263935340acb7)
(cherry picked from commit a7fa2e592e2e579e5acdb903dba83fc074ebc215)
(cherry picked from commit a9d5701b034ed2d9771b3f0943e1add00741d7cd)

Change-Id: I2aed3db568b6fdc487bf99e2c5dd123206736fda