floss: Reset discoverable mode on start

LibBluetooth stores the scan mode on the disk and restores it on start.
Before, Floss actually relies on set_connectable_internal to reset both
connectable mode and discoverable mode together. However, they are now
decoupled by https://r.android.com/3062070 so if discoverable mode is
restored by LibBluetooth, Floss would keep it as is until someone
modifies the discoverable mode.

This patch simply resets the discoverable mode to make sure we have a
clean state on start.

Bug: 339810844
Tag: #floss
Test: mmm packages/modules/Bluetooth
Test: manual btclient: adapter discoverable on 60; adapter disable;
                       adapter enable; adapter show;
                       Device is not in discoverable mode
Test: bluetooth_AdapterSAHealth.{sa_adapter_suspend_resume_test,
                                 sa_basic_test,
                                 sa_eir_max_name_size_test,
                                 sa_dbus_api_tests,
                                 sa_adapter_discoverable_timeout_test,
                                 sa_default_state_test}.floss
Flag: EXEMPT, Floss-only changes
Change-Id: Ia59b6fb0a5e7731ecd7553317bcfb4e0dd00b132