Add prepareToDisconnectExternalDevice.

When an external device is disconnected, some HALs rely on the early
disconnect indication to immediately abort any active read/write
operations on drivers to avoid problems with HW interfaces. In that
case, APM will broadcast the device disconnection before the streams are
closed. However, in AOSP AIDL HAL, the device may not be disconnected
when there are active configurations. In that case, a new way to notify
the HAL to prepare to disconnect external device is required.

Bug: 277955540
Test: atest audiopolicy_tests
Test: connect/disconnect USB device
Change-Id: Ie5868ac5efedb3dbdd8b4c2e3225c0c4085806a1