Retry when starting process from appZygote

We only retry when the original zygote process is dead.
Also we don't kill the dead process again.

Bug: 361799815
Test: manual - Launching new chrome tab after killing chrome_zygote
Flag: android.os.app_zygote_retry_start
Change-Id: Iad42ae4f8763fc06a2f2f183da934ee8ed536a58