Implement LauncherLayoutSnapshot pulled atom callback.

This change will pull only the workspace items but not the launcher settings such as grid layout etc. Future CLs will address it.

Bug: 181703659
Test: Manual-`statsd_testdrive 10108` http://gpaste/4894958784872448
Change-Id: I0e7d7da62034c411edb26ff9205a81b0e2e891a0
6 files changed