ConnectivityService: make log configurable

Make log of ConnectivityService configurable by system property.

Two levels:
VERBOSE: whole VDBG log.
DEBUG: selected necessary log for debug purpose.

Relevant log can be enbled in either way:
1. use adb command at run time.
2. config init.xx.rc file at compile time by adding.
on boot && property:ro.build.type=userdebug
    setprop log.tag.ConnectivityService DEBUG

Bug: 117632924
Change-Id: I43cc84878c64c5b448853c7393393a02262afd15