Allow Linux users to build Android even if Python 2 is their default Python

After this commit, users who have the `python` executable as Python 2 can do simply:

    cd android/
    ./build-all.py
    ./build.py

Previously, the following would fail:

    python3 build-all.py

since build-all.py did subcalls to `python build.py`.

Remove install-all.py as it is redundant with `build-all.py -deploy`,
instead of fixing it as well.

Introduce argparse since we are separating argument parsing out of
the `main()` function.

Tested in Ubuntu 16.04, behaviour should be unchanged for Windows.

download_assets.py

@@ -1,3 +1,5 @@
+#!/usr/bin/env python3
+
 import sys
 from urllib.request import urlretrieve
 from zipfile import ZipFile
@@ -26,4 +28,4 @@ print("Extracting assets")
 
 zip = ZipFile(ASSET_PACK_FILE_NAME, 'r')
 zip.extractall("./")
-zip.close()
+zip.close()