Change return type of tell and tell_frac to uint32_t.

The bit accounting functions aom_reader_tell() and aom_reader_tell_frac()
 return the number of bits and 1/8th bits respectively.
This patch changes the return type from ptrdiff_t which is signed to
 uint32_t which is unsigned.
The size_t type is not used since we only care about the number of bits
 or 1/8 bits per entropy coder context and we don't expect to code more
 than 512 megabits per tile.

Change-Id: I84a119d1f52829dcbdb66a92656eacca06e42b11