Skip screenshots when user storage is locked.

Screenshots are written to shared storage, which isn't available
until after the user unlocks their device for the first time.  So
instead of showing a misleading animation and then the "failed to
persist" notification, skip the screenshot completely when storage
is locked.

Bug: 27208608
Change-Id: I654c6688f68b2395f6e6065aa716551948f302b3