Runtime resource overlay, iteration 1.

Runtime resource overlay allows unmodified applications to appear
as if they had been compiled with additional resources defined. See
libs/utils/README for more information.

This commit is the first iteration of runtime resource overlay. It
provides the actual overlay modifications and loading of trusted overlay
packages (ie residing in /vendor) targeting framework-res.apk.

This commit loads exactly one overlay package. The overlay,
if present, must target framework-res.apk and be located at
/vendor/overlay/framework/framework-res.apk.

Change-Id: If26ee7754813004a96c043dba37fbe99fa3919db