Update the NNAPI delegate to allow user to specify accelerator by name.

  - If an accelerator can be found based on the user provided name, the
  delegate will try to compile and run the model on the selected
  accelerator. An error will occur if the accelerator could not be found..
  - It is the user's responsibility to handle errors from a specific
  accelerator.

Bug: 112661902
Test: mm
Test: Run NNAPI benchmark with driver names specified.
Change-Id: Ie79d0e581ad269e69f64dd1c9bfaed812940d821