Add trunk stable flag for improving routing and volume management

The flag can be used to guard the APIs to improve routing and volume
management in order to take user id / uid into account

Bug: 270049944
Test: m -j, presubmit
Flag: android.media.audiopolicy.volume_group_management_update

Change-Id: I4ef597983753a61ed7b02c0dcf478dc8414f0ccf
Signed-off-by: François Gaffie <francois.gaffie@renault.com>