prepare-release.py commits versionCode bump

when uprev is True
Also change versionName in repo to just say "dev" to avoid
confusion about old versionNames in master

Also dropped the "tagged_" prefix of the tags, so it's
just "releases/*" instead of "tagged_releases/*"

Change-Id: Ie3f309746bc621224e2e82a34f0ad3793292ba4b