Use proper string formatting for the "Active device" string

Updated testContextMock() how it checks the bluetooth_connected string.

Bug: 72317067
Test: Unit test updated:
      make ROBOTEST_FILTER=BluetoothDetailsHeaderControllerTest \
                           RunSettingsRoboTests
      Manual: two headsets and switching the active device
Change-Id: I3db178d71543e4dfa437544350c58241860ae703