use /sdcard/i2pd at android

FS.cpp

@@ -54,6 +54,9 @@ namespace fs {
     dataDir = (home != NULL && strlen(home) > 0) ? home : "";
     dataDir += "/Library/Application Support/" + appName;
     return;
+#elif defined(ANDROID)
+	dataDir = "/sdcard/" + appName; // TODO: might not work for some devices
+	return;
 #else /* other unix */
     char *home = getenv("HOME");
     if (isService) {