update_engine: Break UpdateEngineDaemon into their own implementations

It looks like Android and ChromeOS have completely different
implementation of the daemon. So instead of polluting the source with
USE_* flags, just break it into their own implementation files. At the
very least this allows deprecating USE_BINDER and USE_OMAHA flag
completely.

BUG=chromium:978672
TEST=unittest, cros flash two times.

Change-Id: Ia5c4f9274e275a2c1ba9334111b694514914a475
Reviewed-on: https://chromium-review.googlesource.com/1674583
Tested-by: Amin Hassani <ahassani@chromium.org>
Commit-Ready: Amin Hassani <ahassani@chromium.org>
Legacy-Commit-Queue: Commit Bot <commit-bot@chromium.org>
Reviewed-by: Amin Hassani <ahassani@chromium.org>
diff --git a/libcurl_http_fetcher.cc b/libcurl_http_fetcher.cc
index f469435..d39351c 100644
--- a/libcurl_http_fetcher.cc
+++ b/libcurl_http_fetcher.cc
@@ -269,11 +269,11 @@
     } else if (base::StartsWith(
                    url_, "https://", base::CompareCase::INSENSITIVE_ASCII)) {
       SetCurlOptionsForHttps();
-#if !USE_OMAHA
+#ifdef __ANDROID__
     } else if (base::StartsWith(
                    url_, "file://", base::CompareCase::INSENSITIVE_ASCII)) {
       SetCurlOptionsForFile();
-#endif
+#endif  // __ANDROID__
     } else {
       LOG(ERROR) << "Received invalid URI: " << url_;
       // Lock down to no protocol supported for the transfer.