Mount default encrypted devices at boot

If userdata is default encrypted, we should mount it at boot
to avoid bringing the framework up and then down unnecessarily.

Needs matching vold changes from
 https://googleplex-android-review.googlesource.com/#/c/412649/

Bug: 8769627
Change-Id: I4b8276befd832cd788e15c36edfbf8f0e18d7e6b