Check property for sdcardfs usage

We should check for external_storage.sdcardfs.enabled before deciding
that the system is using sdcardfs.

Test: Run on device not using sdcardfs, but with sdcardfs present in
      kernel
Bug: 160727529
Bug: 181819712
Change-Id: I399348ddd01349fe6c2d5e8365f52eedb9752c90