Update offlinify_docs.py to be more robust.
Update offlinify_docs.py to be more robust.

- Uses beautifulsoup to add css link to each page.
- Removes existing stylesheets.
- Fixes all links to work when opened from filesystem.

Test: Manual
Change-Id: If957762089f9a37a1b38b8417fe2965cb54347a3

androidx/androidx,androidx/androidx,androidx/androidx,AndroidX/androidx,AndroidX/androidx,AndroidX/androidx,AndroidX/androidx,androidx/androidx,AndroidX/androidx,AndroidX/androidx,androidx/androidx,AndroidX/androidx,androidx/androidx,androidx/androidx,AndroidX/androidx,androidx/androidx,androidx/androidx,AndroidX/androidx,androidx/androidx,AndroidX/androidx