Storage: Add secondary storage support for app installation

Add secondary external storage APIs for app installation. And
change the logic to allow app to be installed into external
storage.

Change-Id: If2f67af7508d020c1d041bb955e3d55136a34acc
Hide new methods instead of modifying the public API

Change-Id: I8a1b316f70a0dd76eb6c900d9c1a74e8e1765af3