When passing a size_t via binder, use 64-bits unconditionally

64-bits is almost always over-kill.  But it's easier and cleaner to change
the binder code to be accurate, than to rename all the the API parameter
types to be 32-bit.

Bug: 12381724
Change-Id: Ib8f198d814a2027760ef24e9e3feacee21a973b1