Convert findleaves.py to python 3

Kati special-cases findleaves.py to an internal
C++ implementation, so I don't think this python
script is even used. Update it anyways, and
test by manually running findleaves.py.

Bug: 203436762
Test: ./build/make/tools/findleaves.py --mindepth=2 --dir=build/make/target Android.mk
Change-Id: If27cc37d8f673388c0e31b9491e792b98ca59da4