If someone writes an object into a BagOStuff, they typically expect that
later changes to the object will not affect the value stored.
MultiWriteBagOStuff's async write handling was violating this
expectation, which is potentially causing T168040.
Bug: T168040
Change-Id: Ie897b900befdc8998614af06f9339cd07665703e
Similar to what WANObjectCache and CachedBagOStuff are already doing.
Also add missing tests for WANObjectCache (similar to those for CachedBagOStuff).
Bug: T167465
Change-Id: I1a0c9324726aa6a1b221def985773b1b819181fd