Add trunk stable flag for removal of legacy stream type

The flag can be used to guard the APIs for replacing volume / strategy
management per stream to per port id.

Bug: 317212590
Test: m -j, presubmit
Flag: EXEMPT flag definition
Change-Id: I79f2aa8bfca6d80fb8af94809e9ddaf35c1e236e
Signed-off-by: François Gaffie <francois.gaffie@renault.com>