[SP11] add dumpsys support to dump stats inside providers

Sample output is like below:
  Stats Providers:
    OffloadController Xt:
      NetworkStats: ...
    OffloadController Uid:
      NetworkStats: ...
    ImsPhoneCallTracker Xt:
      NetworkStats: ...

Test: adb shell dumpsys netstats (--uid)
Bug: 130855321

Change-Id: I968006de83e13e66bb3359135f0237daf7252bd2