floss: Support multiple advertising sets

The command, 'advertise on', start an advertising sets with default
parameters and data. This patch allows this command to be executed
more than one time to start multiple advertising sets.
A potential deadlock between btclient command handler and D-Bus
callback while stopping multiple advertising sets is also fixed
in this patch.

Bug: 233128828
Tag: #floss
Test: build.py --target test
Test: manual test with btclient
Change-Id: I68c50670e9101ce0618516a14f75e7d10e8a301a