Let read_nz_map() etc. output to uint8_t*.

Temporarily introduced coeff_is_byte_flag as an argument of
get_nz_count() and get_nz_map_ctx() to handle different types of input
coefficients. It helps to not duplicate too many functions.
If possible, unify types to uint8_t* later.

Change-Id: Idbbe67b7ec563f8c9299daa0fa7d05e15c8295e6