luna-prefs: Use WEBOS_DISTRO_API_VERSION

Have luna-prefs always return WEBOS_DISTRO_API_VERSION for
PRODUCT_DEVICE_NAME_PRODUCT_LINE_VERSION and
PRODUCT_DEVICE_NAME_PRODUCT_BROWSER_OS_VERSION instead of setting its
own values for them.

Open-webOS-DCO-1.0-Signed-off-by: Herb Kuta <herb.kuta@lge.com>
Change-Id: I123ff3d553d7fc9e5f9c5f2ec95342e8e65eb631
Reviewed-on: https://g2g.palm.com/1815
Tested-by: Herb Kuta <herb.kuta@lge.com>
Reviewed-by: Martin Jansa <martin.jansa@palm.com>
Reviewed-by: Build Verification
Reviewed-by: Susan Montooth <susan.montooth@lge.com>