Make install_hooks check if git hook is available

When git hook isn't available, we can still have
install_hooks.sh copy the commands to .git/hooks. However, this means
that the hooks will not remain up-to-date as this repository is updated,
so the user is warned.

Bug: b/187094215
Change-Id: I7401b78d7e6afa12fec691efed7c21b62d987422