Add logging to NetworkStackClient

The current logging relies on the system log that may rotate. This is a
problem to investigate bugs where the network stack service is not
started. Add a SharedLog to NetworkStackClient and dump it in
ConnectivityService (for lack of a better place to dump it).

Bug: 124307802
Test: Booted, dumpsys connectivity shows the log
Change-Id: If596451d0613645f68a8a32acb032eea76ea506c