Open files in source_locator with encoding utf8

Using utf8 encoding type to prevent the encode
problem of the unicode content case.

Bug: 140313558

Test: 1. Patch aosp/1111537.
      2. atest aidegen_unittests --host and it should all pass.

Change-Id: If2ef052c4620a9d928ff8ab2bd58a619742957d6
1 file changed