Minor update to decrypt device routine for Nexus 6P decrypt

Move property setting of ro.crypto.state to after mounting any
needed partitions so that we can use this property as a trigger
for running services in init. Check to see if the property is
already set so that we do not set it and sleep multiple times if
the user enters an incorrect password.

Change-Id: I55558c5e227377381101b6305569d9eb31040790