Improve binder calls stats sampling.

- Decouple detailed tracking and sampling. Detailed tracking now control
which data wwe should track (e.g. exception, max reply size, ...),
sampling controls the % of requests to track
- Do not create a CallStat object just for the sake of tracking the call
count. Most of the keys have a really low call count (60% of keys below
10 call counts, 90% below 100 calls). When data sampling is enabled, we
can save lots of memory.

Test: unit test
Change-Id: If81b38023a76a8b1e07e655f9ecd8a098d401df4