Backendlib should provide installed_time, installed_size, app_size, data_size

BUG=XWALK-2459

backendlib should set those field in pkgmgr detail info structure.

pkgcmd -s -t wgt -q -n ${pkgId} # should return this data

This information may be present in other apps to user if he/she want to install app.
this class is supposed to be read-only, installation time should be provided in the constructor.