Cleanup logic in KillZramBackingDevice

Since this function is used in userspace reboot, we need to be more
diligent with error handling, e.g.:

* If init fails to read /sys/block/zram0/backing_dev, then fail and
fallback to hard reboot.
* Always call swapoff.
* Always reset zram.
* Tear down loop device only if zram is backed by a loop device.

Test: adb reboot userspace
Bug: 153917129
Change-Id: I4709da1d08cf427ad9c898cfb2506b6a29f1d680