Add (partial) support for Windows long paths

* Update android::base::utf8::open/unlink to support Windows long
  paths

* Add android::base::utf8::fopen, also with support for Windows long
  paths

* Upcoming CLs will add additional APIs to support additional use cases

Test: Added tests to utf8_test
Bug: 38268753
Change-Id: If72af327f3487766f5370a2f43ee9cabd4a8a810