Themes: Build icon cache when applying icons

The theme service will schedule building of the icon cache when
an icon pack is applied.  This will allow the icon cache to be
populated immediately after icons are applied rather than build
it up as icons are requested.

Change-Id: I453eda5f6298ff79e22827c622a0eddcab9c721d
Merge "Themes: Build icon cache when applying icons" into kk44