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
c259e615e6