Bring back the SECONDARY_STORAGE variable

This variable is a requirement in KK as newly introduced APIs
such as Context.getExternalFilesDirs uses it to retrieve
secondary storage locations on the device.

Change-Id: I17708c93498da5979c5742cad32232a34490bf5f
(cherry picked from commit 6595112a242dc49c4a485da819e39f5d539fc331)