libsnapshot: Add ISnapshotManager.

This is the public interface of SnapshotManager.
SnapshotManager then inherits from it.

Test: compiles
Test: vts_libsnapshot_test
Test: update_engine_unittests
Bug: 148956645
Bug: 153555889
Change-Id: Ib19c038f9bb41dd8d27f72ff6a24c231cd4c5a76