ActiveDeviceManager: Fix handling disconnect of a streaming set member

This patch fixes issue when after disconnecting LeAudio device while
stream, music was transfered to the Speaker.

ActiveDeviceManager, while handling disconnection of the first set
member, incorrectly assumed that another set member is a fallback
device. Then, ActiveDeviceManager instructed LeAudioService to remove
itself from being Active device, but also set flag `hasFallbackDevice`
which later cause the problem

Bug: 367213572
Bug: 374320313
Test: atest ActiveDeviceManagerTest
Flag: com.android.bluetooth.flags.adm_fix_disconnect_of_set_member

Change-Id: I6ea853a2565f9ce56ef8eaa6df8d4d515c065a8d