floss: Add advertise set-interval command

`advertise set-interval` command changes the advertising interval
of all advertising sets registered.
This patch also introduces struct AdvSet to keep information of an
advertising set, so that btclient has the capability to alter
current parameters and data.

Bug: 233128828
Tag: #floss
Test: build.py --target test
Test: btclient advertise set-interval 400

Change-Id: I2c1db17399215c1d06180a82e326743c47208c2d