WebView: use Asset_read() instead of Asset_openFileDescriptor()

openFileDescriptor doesn't work for assets which are compressed inside
the APK.

src/api-impl-jni/AssetInputStream.c

@@ -0,0 +1,30 @@
+#include <androidfw/androidfw_c_api.h>
+
+#include "AssetInputStream.h"
+
+static gssize asset_input_stream_read(GInputStream *gstream, void *buffer, gsize count, GCancellable *cancellable, GError **error) {
+	return Asset_read(ATL_ASSET_INPUT_STREAM(gstream)->asset, buffer, count);
+}
+
+static gboolean asset_input_stream_close(GInputStream *gstream, GCancellable *cancellable, GError **error) {
+	AssetInputStream *stream = ATL_ASSET_INPUT_STREAM(gstream);
+	Asset_delete(stream->asset);
+	stream->asset = NULL;
+	return TRUE;
+}
+
+static void asset_input_stream_class_init(AssetInputStreamClass *class) {
+	class->parent_class.read_fn  = asset_input_stream_read;
+	class->parent_class.close_fn = asset_input_stream_close;
+}
+
+static void asset_input_stream_init(AssetInputStream *self) {
+}
+
+G_DEFINE_TYPE(AssetInputStream, asset_input_stream, G_TYPE_INPUT_STREAM)
+
+GInputStream *asset_input_stream_new(struct Asset *asset) {
+	AssetInputStream *stream = g_object_new(asset_input_stream_get_type(), NULL);
+	stream->asset = asset;
+	return &stream->parent_instance;
+}