Themes: Add versioning to the theme feature

As new features and enhancements to the theme apk format are added
we will need a way for themes to specify which version of the theme
format they are targeting.  Exclusion of the theme feature in a
theme apk will imply v1.

Change-Id: I0016d58341ca3144bb4a4faa16663ab29439ab24
Add an init.d script to facilitate rolling out a one-time thumbnail purge

Change-Id: I450f69bce39bc070b55b4a244e57d09015501fa0