Revert "Themes: Build icon cache when applying icons"

This reverts commit 7bb802a4.

This is not thread safe.  It is possible for an icon to be in the
middle of being composed when another process requests it.  That
process would not be able to load the image.  In Trebuchet, the
icon would be displayed as the standard "android" icon since it
would not be able to load the icon.

Change-Id: Id9bc38296cc02c376f9ea66565babc00e8dbf634