issue #2434: add android_generator and generate cocos2dx, extensions and TestCpp's android.mk files
add PathUtils.py for python library.
#closed #2434: refactor LocalVarToAuto.py  to use the PathUtils and fix some errors
closed #2434: change name "android_generator" to "android_mk_generator
issue #2434: Delete unused method dirs and find_files in Android.mk template
closed #2434: Use the origin android.mk files instead use templates
Merge pull request #3457 from boyu0/iss2434_gen_android_mk

closed #2434: Add android_mk_generator.py to generate Android.mk