libsnapshot: Move ImageManager creation to DeviceInfo.

This refactoring moves ImageManager creation out of SnapshotManager,
where it had always been rather awkward, and into IDeviceInfo so it can
participate in dependency injection.

The "first-stage init" state is now part of IDeviceInfo as well.

Bug: 187396878
Test: vts_libsnapshot_test
      libsnapshot_fuzzer_test
Change-Id: Ic4b3e93527cc074ec18c0c26440dbdb2504d0d5e