Miscellaneous B2G viewer fixes [WIP]

After merging some PRs, the B2G viewer had some issues displaying documents properly. This PR fixes the following B2G viewer problems:

1. Stop initializing the hand tool in the B2G viewer: the B2G viewer stopped rendering pages because `toggleHandTool` was `null`. We now disable the hand tool if the toggle button is not available.

To do:
- [ ] B2G viewer is only rendering when scrolling down a little bit. It should render immediately. This seems to have regressed in #3978. /cc @brendandahl