logd: use coalesce instead of merge (cleanup)

(cherry pick from commit 95ff2482176e607ea7c77463c75c5443030fe2b2)

- switch to coalesce instead of merge in naming of functions
  and variables. Confusing since we also to merge-sorts and
  other activities in the logger.
- define maxPrune rather than using a number in the code path.

Bug: 24511000
3 files changed
tree: f441e982ee0700fb55cc02713b428eed8919de85
  1. liblog/
  2. logcat/
  3. logd/
  4. logwrapper/