Get timestamp from ro.build.date.utc

If you upload a file by hand on the server, the creation date of the file
will be newer than the compilation date of the ROM and CMUpdater
will believe that is a new version because CMUpdater compare the date
contained in the build.prop (ro.build.date.utc).

Change-Id: I6a7afda362b7a7e4f12762282fc71955caee74e4