service: Support base switches::kV

This is a libbase built-in command-line flag to set log verbosity,
so that the VLOG macro does the right thing. This CL enables that for
the daemon. To enable VLOGs with a given verbosity level and below, do

bluetoothtbd --v=<level>

Change-Id: I128a42c52425c37640c016c3c6158419cc1c0f66