hodec2: add ho_candidate.{current,target}.free_tch

Store the number of free lchans and the min-free-slots settings in
ho_candidate, instead of figuring those out in various places, to make
it easier to read.

Prepare for upcoming patch which also requires these values to fix a
bug.

Change-Id: Ie6ca5af5e8d0ebb8deaaaa637e2728008ecba517
1 file changed