Improve logging for NetworkStatsObservers

In order to have better logging to gather information
about the callers. This change includes:
 1. Add pid and package name into RequestInfo for better logging.
 2. Print current observers in dumpsys section.
 3. Enable register/unregister usage request logging by default.

The log added in the dumpsys section looks like:
Stats Observers:
  RequestInfo from pid/uid:2001/1000(android) for DataUsageRequest [...] accessLevel:3
  ...

Test: NetworkStatsObserversTest
Bug: 229103088
Change-Id: Ide68776dc279610e8dee0efd924b59a1fac2b7b6
(cherry picked from commit c259e615e68aacc3bc47b729de03449b5798a1b4)
Merged-In: Ide68776dc279610e8dee0efd924b59a1fac2b7b6
3 files changed