libsnapshot: Switch to IPropertyFetcher.

vts_libsnapshot_test uses IPropertyFetcher, as does liblp, but
libsnapshot doesn't. Fix this by switching relevant cases in
utility.cpp.

This also changes SnapshotTestPropertyFetcher from a mocked class to a
pseudo-implementation. This is useful because sometimes we want the
actual device's properties and sometimes we don't, and this setup is
more flexible.

Bug: 208944665
Test: vts_libsnapshot_test
Change-Id: I58621281715b3efb045b6be6f788934fddaa1a0c