[android] Use stub HTTP file source for test runners

platform/android/src/test/http_file_source_test_stub.cpp

@@ -0,0 +1,18 @@
+#include <mbgl/storage/http_file_source.hpp>
+#include <mbgl/storage/resource.hpp>
+
+#include <mbgl/util/async_task.hpp>
+
+namespace mbgl {
+
+class HTTPFileSource::Impl {};
+
+HTTPFileSource::HTTPFileSource() : impl(std::make_unique<Impl>()) {}
+
+HTTPFileSource::~HTTPFileSource() = default;
+
+std::unique_ptr<AsyncRequest> HTTPFileSource::request(const Resource&, Callback) {
+    return std::make_unique<AsyncRequest>();
+}
+
+} // namespace mbgl