Stop relying on /usr/bin/python for SDK builds

Instead, use python2 from $PATH, which will be a prebuilt inside the
build system.

Test: treehugger
Change-Id: I81966e85377c8e9dd87e1f2656f6c0ec6b3a636c

build/tools/mk_sources_zip.py

@@ -1,4 +1,4 @@
-#!/usr/bin/python
+#!/usr/bin/env python2
 #
 # Copyright (C) 2011 The Android Open Source Project
 #