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.

android/uninstall-all.py

@@ -1,3 +1,5 @@
+#!/usr/bin/env python3
+
 # Remove all examples from connected device(s)
 import subprocess
 import sys
@@ -64,4 +66,4 @@ if answer:
         print("Uninstalling %s (%d/%d)" % (app, CURR_INDEX, len(APP_NAMES)))
         subprocess.call("adb uninstall %s" % (app))
         CURR_INDEX += 1
-    
+