Change ASSISTANT_SESSION_UNKNOWN ID

ASSISTANT_SESSION_UNKNOWN is a default value that is not expected to be
used, therefore it does not need to use a reserved value.

Test: Tested locally
BUG: 162956422
FIX: 162956422
Change-Id: Ifa6ae42eb883915d9db3e83566ac71cf02e007f7
1 file changed