Config deserialization: allow negative gain

Audio policy configuration deserializer did not support negative gains
in audio profiles.

As those gains are most always attenuation (negative) this mean that
profile can not currently contain gains.

Test: policy sysdump show negative gain
Test: play music and assistant work on speaker, skylab
Bug: 68229318
Change-Id: I7d72a0dd464aa77847d7e3ec0cfb4dcf7ac25b24
Signed-off-by: Kevin Rocard <krocard@google.com>