Sanitize thread data sharing in NetlinkListener

This is a followup to a fix for an asan hit (aosp/790281) suggested by
huangluke@. Made a few variables const, added explicit lock annotations
and, last but definitely not least, made mWorker start after the entire
class is initialized.

Test: atest netd_integration_test
Change-Id: I08ccfb3a2e7c3f160a5e04ab66007d468aaa140c
3 files changed