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: Icc8f57b61d0b23a84e7244566a5403dd221cc0a8
Signed-off-by: Jorrit Jongma <git@jongma.org>