base: Fix the problems for runtime overlay.

 - There is no need to make the string blocking when adding the asset path,
   as it will be made by the resources.
 - After adding the overlay path, it also needs to update the string blocks.
 - Scan all the sub folders for framework overlay res.

Change-Id: Iaad019111ae364c319e58dce57dbf4647b38d4c3