[gatekeeperd] fix issue with SW->HW upgrades

If the handle version is 0, there's no hardware_backed flag
meaning hardware backed handles will be attempted against
the soft impl. Ensure we don't try to read from hardware_backed
unless the version is > 0.

Bug: 21090356
Change-Id: I65f009c55538ea3c20eb486b580eb11ce93934fc