[QS] Add secondary label to Hotspot tile

Added info to callback and leveraged WifiManager API to get the number
of connected devices. There's some minor state cleanup done and the
fire callback mechanism was slightly updated.

The secondary label is updated based on the connected devices & enabled
state.

Screenshot: https://screenshot.googleplex.com/jHMKmg2nO08

Test: manual
Bug:68058038
Change-Id: Iafcebd3476d2c13e25939b45e78f32a5bd24cb68