Allow OMX pointer types to be forced to 32 bits

Enabled by #define.  Is used for 64 bit compilation to
force structs to use 32 bit pointer values for eventual
IPC to 32 bit OMX codecs.

May change later if we support 64 bit OMX codecs.

Change-Id: I0d12f86774f6f7ce9c39c8abea5d51988a7eef11
Signed-off-by: Andy Hung <hunga@google.com>