PandoraServer: Avoid completing grpcUnary responseObserver twice

Due to a suspension point at the end of the withTimeout block,
a cancelation could happen after calling onCompleted
on the grpc responseObserver.
This cancelation would throw an exception ending in the catch block
with an already completed responseObserver.
Calling onError on an already completed responseObserver ends in
an unhandled IllegalStateException ending the MonitoringInstrumentation.

To solve that we move the suspension point before calling onCompleted.

Bug: 244615429
Test: atest pts-bot
Change-Id: Ia38a3f0a1baf88df081b49622809cca6f61e1e12