[DPM] Management and retrieval of network logs

This CL follows up on ag/1530343 and adds:
1) Various network events.
2) Retrieval method in DPM and APIs in DeviceAdminReceiver.
3) Extension of NetworkLogger and it's NetworkLoggingHandler.

Test: runtest --path frameworks/base/services/tests/servicestests/src/com/android/server/devicepolicy/NetworkEventTest.java
Bug: 29748723
Change-Id: I42a1a477e7c75c109a3982f809c22732b814e8b2