Add tests for ActiveDeviceManager LeAudio->A2dp

When switching a set of earbuds from LeAudio to A2DP this is the same as
disconnecting from LeAudio device and connecting to A2DP device so the
new tests are verifying the behavior of the ADM when disconnecting one
or both LeAudio devices.

Bug: 278761210
Test: atest BluetoothInstrumentationTests
Tag: #feature
Change-Id: Ie708737c760862af8d0b100134a53c917077b124