Introduce ImmutableVolumeInfo.

Now that we're caching VolumeInfo objects, make sure that they cannot
easily be changed without the cache knowing about it. Use immutable
versions when we can.

Bug: 323574186
Flag: EXEMPT refactor
Test: N/A
Change-Id: Ib8a00e7307e6466d64a8b3fdfc5a2272a1f4d308