Check property for sdcardfs use

Sdcardfs is only used if present in the kernel, and
external_storage.sdcardfs.enabled=0. For compatibility, this property
defaults to 1.

Bug: 155222498
Test: mount|grep "type sdcardfs" should find nothing after boot complete
      if external_storage.sdcardfs.enabled=0
Change-Id: Ide648405c260ac46d07877c3a7b78a8bee48e09a