libsnapshot: Add SnapshotManagerStub.

Avoid crashes in update_engine. When SnapshotManager
should not be used (e.g. Virtual A/B is not enabled),
use the stub instead.

Bug: 148956645
Test: update_engine_unittests
Change-Id: Id524a1dfe5d085d439fc5492d4b7f72ee583c17e