Adjust ro.modversion and add ro.omni.version

Both properties are similar, but ro.modversion is prefixed with
OmniROM-. custom_ and omni_ are removed from the TARGET_PRODUCT
portion of the identifier (other product types will still show)

Change-Id: Ia08d337f7132c22eb71244e886dbe4d5ba24d158
Signed-off-by: Jorrit Jongma <git@jongma.org>
Merge "Adjust ro.modversion and add ro.omni.version" into android-4.3