Respect `namespace mnt`

This fixes the regression of aosp/2153354.
SwitchToMountNamespaceIfNeeded() is supposed to switch between "default"
mount namespace and "bootstrap" mount namespace. But it shouldn't affect
services with its own mount namespace.

Bug: 239882455
Test: make a hal service start in a separate mount namespace
    using 'namespace mnt' in its definition.
    see the mount namespace of the service process
    $ cat /proc/$(pgrep -f myservice)/ns/mnt
Change-Id: I8c80eaec723241c405f48980b9e88640123c43ad