libsnapshot: Fix libsnapshot_fuzzer_test.

Rather than fix this up to work in all configurations, just
force-disable snapuserd testing for these tests.

This patch also adds a "UseUserspaceSnapshots" helper to IDeviceInfo so
we can simplify some checks in snapshot.cpp.

Bug: 208944665
Test: libsnapshot_fuzzer_test
Change-Id: I215635645d49ad21e80af7fe084e25669c64110c