Stop calling fsck on shutdown by default

Instead, run the command only when sys.powerctl is set to
"shutdown,userrequested". This way, we can avoid running
fsck when shutdown is triggered due to a low power state.

This is a follow-up CL for http://r.android.com/158525.

Bug: 21853106
Change-Id: Ie57c23cd25162cc2a8726f876a9ba212080105fb