build: make fs_config_generator.py python2 explicitly

Test: run script in Fedora 31 with (default) python3 interpreter.

```
$ env python --version
Python 3.7.9
```

Change-Id: Ib4e68c5df54b092380e9fca473cd0dbbc0c18db7

tools/fs_config/fs_config_generator.py

@@ -1,4 +1,4 @@
-#!/usr/bin/env python
+#!/usr/bin/env python2
 """Generates config files for Android file system properties.
 
 This script is used for generating configuration files for configuring