AudioSystem: fix missing audio port callbacks after audioserver restart

Commit ce3d1106 dropped the enablement of audio port and volume group
callbacks after registering the audio policy client when reconnecting
audiosystem to a new instance of audio policy service.

Bug: 378032953
Test: make
Flag: EXEMPT bug fix
Change-Id: Ia20bbe206b6073c37b4ba07faf3b906abd52b549