Don't delay starting sessions.

It used to be the case that a volume could be mounted before its
corresponding user was unlocked, and in that case we needed to hold off
on starting sessions. That is however no longer the case, so we should
be able to start all sessions immediately.

Bug: 137890172
Test: atest AdoptableHostTest
Change-Id: Id4ea069631e58b244e401609c22aef485133578c