Zero SSRC are technically invalid, but the code accepts them as valid
and therefore sets up things like crypto contexts for zero SSRCs.
Consequently it's possible to have a zero SSRC first in the list of
SSRCs and other valid SSRCs later in the list, which means we can't use
the presence of a non-zero SSRC first in the list as a flag to determine
whether SSRC tracking is in use or not. Use an explicit flag instead.
Change-Id: I88736e5d6b0f66c58f8d675137231760951e7610