Add secondary external storage support.

When SECONDARY_STORAGE is defined by the hardware configuration,
include those paths in the new getExternalCacheDirs() and
getExternalFilesDirs() APIs.  Give secondary volumes valid state
until vold reports back.

Bug: 10330128, 10330229
Change-Id: Ifa55bfda47fd7c750f462dd26c98792ad462ab91