libvndksupport: Add android_is_in_vendor_process()

In preparation for cleaning up conditionally compiled VNDK libraries,
we need a function that determines if the current process is running in
the system process or the vendor process.

Bug: 119423884
Test: Add a dummy VNDK library that prints different string in system
      vs vendor process.  Use the library in different processs and
      check the output.

Change-Id: Ic00927d8eadd2a21fcba9cb64cc93eaadd5bdd2d