Auto-encrypt drive at startup

Modify fs_mgr to unmount encryptable drives after test mounting them and
then trigger an auto-encrypt via the init script

Needs matching vold changes from
 https://googleplex-android-review.googlesource.com/#/c/414200/

Feature is limited to list of serial numbers with this change

Bug: 11985952
Change-Id: I84f85a258b6a7e9809467c9149249302e203c41b