Workaround 64 bit devices that don't have zygotes yet.

This can be fixed by tweaking their BoardConfig as well,
but that leads to all sorts of other problems.

For now, fall back to the primary zygote if the secondary
is unavailable. This will fail if shared libraries for the
primary zygote ABI are unavailable, but that won't be the
case just yet.

bug: 13970103
Change-Id: Ifa107e43ca95023d57a0d9c641e4fddaccd9a888