vold: only allow emmc_optimized on eMMC storage

The emmc_optimized encryption flag is specifically designed for the
limitations of inline encryption hardware that follows the eMMC
standard.  It isn't appropriate to use on other types of storage.
So, make vold enforce that it's not used on other types of storage.

Bug: 160639344
Test:
  - Enabled emmc_optimized on Cuttlefish and verified it no longer boots
  - Using a modified version of this change, verified that
    IsEmmcStorage() works as expected on various devices including
    Cuttlefish, Cuttlefish booted in GSI image mode, a device with eMMC
    storage, and a device with UFS storage.
  - Verified that VtsKernelEncryptionTest still passes
Change-Id: Ie27b80658db53b1a4207b3cbb4e309d05130812e