Wait for secondary zygote before bringing up the system_server.

The zygote that's responsible for starting up the system server
now checks if there's another zygote on the system, and waits
for it to start up. Also, a few minor clean ups :

- Address a long standing TODO about zygote retries.
- Have functions throw IOException where appropriate and
  wrap them in ZygoteStartFailedEx with a filled in cause.

bug: 14869939

Change-Id: I9e514659b79b3d2c98a4c5f93c0c376843f6c881