sign-launcher: make the help text more automatic
sign-launcher: make the help text more automatic

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 <93465216fd46a440eb04b27d6e009aced8b0c153@google.com>
Tested-by: Mike Frysinger <8f3f75c74bd5184edcfa6534cab3c13a00a2f794@google.com>

GerritCodeReview/git-repo,couchbasedeps/git-repo,lewixliu/git-repo,GerritCodeReview/git-repo,lewixliu/git-repo,couchbasedeps/git-repo