Revert "Themes: Build icon cache when applying icons"

This reverts commit 7bb802a478faaac295ccd8b98a3573701edd9b1e.

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
Themes: Handle processing themes in separate thread

If themes are being processed and the user attempts to apply a theme
they will be forced to wait for themes to finish processing before
the theme is applied.  This patch moves processing of theme resources
into a separate handler/thread.

Change-Id: Ib30a1be75915364c2867df04d63700be63c9f2c5