zygote: respond and wait for reply to --boot-completed.

The zygote handles requests by polling on its sockets, and then handling
one request for each active socket. However, it does so by reading from
a socket via a BufferedReader, which means that if multiple requests are
written into the socket before the zygote gets a chance to read them,
the zygote reads multiple requests into its BufferedReader, it handles
one request, and then never responds to the request that's buffered,
leaving its client stuck waiting for a response that will never happen.

For most requests, this can't happen, because the client will wait for a
response to be sent from the zygote before sending another request, but
this isn't true for --boot-completed until this patch.

Bug: http://b/141767463
Test: forrest runs of apct/text/text_native_test-cloud-tf
Change-Id: I8b7a80abfd9443d98f8cf5aedb7669b82c0cb84a