Activate the last connected BT device

When BT1 connected -> BT2 connected -> BT2 disconnected,
then the sound came from phone speaker before this CL.
With the CL, now the sound comes from BT1.

Bug: 243053736
Tag: #refactor
Test: atest BluetoothInstrumentationTests:ActiveDeviceManagerTest
Change-Id: I1d019bfd117f7e9f13618dfa989c331b33be3f42