DataSync tizen plugin implementation

BUG=XWALK-1061

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Running TCT:
 - install rpms on device:
     - libwbxml2,
     - sync-agent,
     - oma-ds-agent (and start: systemctl start oma-ds-agent.service),
 - copy 'tct-datasync-tizen-tests.wgt' to device,
 - install and run application,

Current TCT datasync passrate: 63/63

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should not return null if callback is invalid of function?