AAPT2: Read config from disk

Implement the todo left from last change to read the contents of the
configuration file from disk. Since this is an operation that may fail
the API was changed to take return a Maybe to indicate errors reading
the file.

Test: unit test for error condition
Test: ran aapt2 optimize with the new code path wired in

Change-Id: I93d532b4a57af9520231225eee4fc5f2b1a046b9