cm: Add a simplified version string

Similar to the ro.build.id/ro.build.display.id distinction, and with
the same rationale; signed builds are intended to be stable by
definition, the release types have different meanings (user vs
userdebug), and the full CM version has too much duplicated information:
the device's name and build date are already present elsewhere in the
same information screen.

For signed builds, remove the duplicated information and the type,
leaving only the actual numeric version and the verbose build identifier,
turning for example "11-20140109-SNAPSHOT-HappyPonies-device" into just
"11.0-HappyPonies". If a simplified display version can't be built,
it'll be the same as the full ro.cm.version

Change-Id: I7d8cccbb3205bde710f0004df0a6bd12c39693f1