vold: support v2 encryption policies

Add support for setting v2 encryption policies when configured in the
fstab (for internal storage) or in system properties (for adoptable
storage), and for installing and evicting the keys for such policies.

v2 policies support the same encryption modes and flags as v1 policies,
but internally they use a more standard, secure, and flexible KDF.  Due
to this, some future features will be supported by v2 policies only.

Bug: 140500999
Test: Configured a device to use v2 encryption policies (applied the
      needed kernel patches and added
      "fileencryption=aes-256-xts:aes-256-cts:v2" to fstab, and set the
      corresponding system properties for adoptable storage).  Wiped
      userdata, booted device and checked logs to verify that v2
      policies were being used.

      Also enabled virtual SD card and formatted as adoptable storage;
      verified it works and that v2 policies were being used on it.

      Also created, started, and stopped a 2nd user and verified their
      keys were evicted.

      Also verified that the device comes up again after rebooting.

      Also verified that a device using v1 encryption policies continues
      to work, both with and without an updated kernel -- including
      stopping a user so that their keys get evicted.

Change-Id: If64028d8580584b2c33c614cabd5d6b93657f608