For devices which are not treble enabled, return the system shell.

For treble enabled devices, still return the appropriate shell depending
on whether the process is a vendor process or a system one.

Test: Manual testing: on a bullhead device, ran test programs from
      /vendor/bin which used popen() and system(). The calls succeeded.

Bug: 65054230

Bug: 64516799

Change-Id: I15dfdbb107cfca7c0f92f337c9bb46b9876eb38e