Networking unit tests: fix some flaky tests

 - less strict regex for SharedLogTest: the subsecond part of the
   timestamp can have 0, 1, 2 or 3 digits.
 - refactor NetworkStatsServiceTest and NetworkStatsObserversTest to use
   waitForIdleHandler facility of ConnectivityServiceTest.
   NetworkStatsServiceTest was using a flaky custom version of
   waitForIdleHandler.

Bug: 62918393
Bug: 32561414
Test: runtest frameworks-net
Change-Id: I634acfb5f4fe1bd5267e3f14b9f645edc32d5d12