Add tests for converting license in androidmk

1. Added unit tests for license conversions in the androidmk tool, by
using a mocked filesystem;
2. Added integration tests in a shell script with a mocked filesystem.

Bug: 205615944
Test: m androidmk
Test: bash androidmk_test.sh
Test: TreeHugger
Change-Id: Id3d1f914190b52b82aadda7ea019192787b839d6