manifest_xml: unify bool & int parsing

We've been overly lenient with boolean parsing by ignoring invalid
values as "false" even if the user didn't intend that.  Turn all
unknown values into warnings to avoid breaking existing manifests,
and unify the parsing logic in a helper to simplify.

We've been stricter about numbers, but still copying & pasting
inconsistent code.  Add a helper for this too.  For out of range
sync-j numbers (i.e. less than 1), throw a warning for now, but
mark it for future hard failures.

Change-Id: I924162b8036e6a5f1e31b6ebb24b6a26ed63712d
Reviewed-on: https://gerrit-review.googlesource.com/c/git-repo/+/256457


Reviewed-by: Michael Mortensen <mmortensen@google.com>
Tested-by: Mike Frysinger <vapier@google.com>