Previously, loader.signalProcess was inconsitently using both root and
container's PID namespace to find the process. It used root namespace
for the exec'd process and container's PID namespace for other processes.
This fixes the code to use the root PID namespace across the board, which
is the same PID reported in `runsc ps` (or soon will after
https://github.com/google/gvisor/pull/5519).
PiperOrigin-RevId: 358836297
19fe3a2bfb