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

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