[android] use shared DefaultFileSource for all maps + OfflineManager

platform/android/src/default_file_source.cpp

@@ -0,0 +1,20 @@
+#include "default_file_source.hpp"
+#include <mbgl/util/logging.hpp>
+
+#include <cassert>
+
+namespace mbgl {
+namespace android {
+
+DefaultFileSource& defaultFileSource(const std::string& cachePath_, const std::string& assetRoot_) {
+    static auto cachePath = cachePath_;
+    assert(cachePath == cachePath_);
+    static auto assetRoot = assetRoot_;
+    assert(assetRoot == assetRoot_);
+
+    static DefaultFileSource defaultFileSource{ cachePath, assetRoot };
+    return defaultFileSource;
+}
+
+} // namespace android
+} // namespace mbgl