Make encryption configurable

Delay mounting encryptable but unencrypted volumes until we can
check the ro.vold.forceencrypt flag, then optionally encrypt.

Requires matching vold change from
    https://googleplex-android-review.git.corp.google.com/#/c/615309/

Bug: 18764230
Change-Id: If22008be8de6a4f3216b349f81ace49be1730314