snapuserd: Remove test dependence on LoopDevice.

LoopDevice requires root, which is an obstacle to running this test in
automation. The test also requires memfd which is not available in our
included glibc. Create an IBackingDevice layer so we can use temporary
files instead on host tests, while keeping the block-device code for
on-device tests, which more closely matches how snapuserd runs.

Bug: 288273605
Test: snapuserd_test
Change-Id: I89b154921b6bbcf8fe213ef7f5c4da4d48322909