Add:port/android:Support gps_status OSD with Android vehicle

Signed-off-by: mvglasow <michael -at- vonglasow.com>
Fix:port/android:Update gps_status OSD when GPS fix is lost

Signed-off-by: mvglasow <michael -at- vonglasow.com>
Fix:port/maemo:Correct debug output when getting attr_position_sats

Signed-off-by: mvglasow <michael -at- vonglasow.com>
Add:core:Add support for GPS status callbacks in navit object

Signed-off-by: mvglasow <michael -at- vonglasow.com>
Add:osd:Update gps_status when status changes but not position

Signed-off-by: mvglasow <michael -at- vonglasow.com>
Fix:port/android:Remove obsolete workaround for gps_status update

Signed-off-by: mvglasow <michael -at- vonglasow.com>
Refactoring:port/android:Remove unneeded debug output

Signed-off-by: mvglasow <michael -at- vonglasow.com>
Refactoring:core:document values for attr_position_valid

Signed-off-by: mvglasow <michael -at- vonglasow.com>