Add API to check for emulated external storage

When the storage is emulated, we don't want to install ASEC containers
to it. This adds the API to check when the external storage is emulated
and uses it to check whether or not to install packages to the external
storage in an ASEC container.

Bug: 3024387
Change-Id: Ia0318aca9e4938a4897deaada5603a4c7c1d0f48