Trebuchet : Fix cellsize for dynamic changes

When the grid size is changed, this logic does not take that into account.
Fix that by allowing the cellWidth/cellHeight to be set in onMeasure.

Change-Id: I58938eea806a527d1cb54786fa045181455fefff
Merge branch 'cm-10.2' of github.com:CyanogenMod/android_packages_apps_Trebuchet into cm-10.2