Fix a bug where cache was double-counted.

Cache was being counted as cache and also as data in the app info page.
This is due to a faulty assumption I made that getDataBytes() and
getCacheBytes() measured distinct areas of storage (in actuality,
getCacheBytes() is a subset of getDataBytes()). This improper assumption
also led to the unit test being incorrect.

Change-Id: I4144d50800f82feaecb7a0a8dff26be3e4f14da8
Fixes: 62182151
Test: Settings robotest & manual