PrepareZramBackingDevice: use loop_control.h

loop_control.h already provides loop device management APIs that we need
here. In addition, this change fixes a subtle race condition between
uevented creating a loop device node in userspace and
PrepareZramBackingDevice accessing it without waiting for it to be
created.

Test: device boots
Test: adb reboot userspace
Bug: 154500256
Change-Id: If80f18c8c337210030a6caf2aec6f7a47472b6fb