[SB][ComposeIcons] Add connectedDevices StateFlow

This change adds a StateFlow in the BluetoothRepository
that flows on the bgDispatcher with updates to the currently
connected bluetooth devices.
This will be used for RA implementations for bluetooth states
in the status bar.

Bug: 414890231
Test: BluetoothRepositoryImplTest
Flag: com.android.systemui.status_bar_system_status_icons_in_compose
Change-Id: I029be765691da37aad0ce02757f0e1ad0ae9ed4c