Rather than display "3.0" all the time and confuse people, extract
the version from the launcher we're signing and display that.
Also reformat the text to follow our current practice: upload the
versioned launcher by itself first, and then later copy that over
the default.
And while we're here, add tips for rollbacks.
Change-Id: I1654425c88e5c67d78879f2f33ad685c59be14dc
Reviewed-on: https://gerrit-review.googlesource.com/c/git-repo/+/319637
Reviewed-by: Xin Li <delphij@google.com>
Tested-by: Mike Frysinger <vapier@google.com>