Replace C++ API by the C API provided by libpac

- libpac will be moved into the Runtime APEX module.
  Use the new stable C API interface provided by libpac
- The change also removes the following debug log when
  error occurs.

  ALOGE("Error Running PAC: %s", ret8.string());

  When ProxyServerV8::GetProxyForURL != OK, ret8 may not
  contain the error message, but the non-ASCII proxy names.

Bug: 121269980
Test: m droid
Change-Id: I0ea0ad7489a23cbc0476dcd66d320f80499f8be1