Themes: Restructure resource cache [1/2]

The new structure is as follows:
/data/resource-cache/
        ├─ theme1_pkg_name
        |     ├─ target1_pkg_name
        |     |     ├─ idmap
        |     |     └─ resources.apk
        |     ├─ target2_pkg_name
        |     ·
        |     ·
        |     ·
        |     ├─ targetN_pkg_name
        |     └─ icons
        |           ├─ hash
        |           └─ resources.apk
        ├─ theme2_pkg_name
        ·
        ·
        ·
        └─ themeN_pkg_name

Change-Id: Id39688c88929733b42368c1f20ef0e25848a3390