[FspCache] Rename ContentCache to CacheManager

ContentCache is actually the underlying files that are being stored.
There will be a number of additional capabilities (e.g. database etc.)
that mean this particularly class is probably more suitable to be called
the CacheManager.

Bug: b/317137739
Test: autoninja -C out/Files chrome
Change-Id: Id53cc4b6d42813a91ac335ffccdc17cd7b8e6e96
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5296825
Reviewed-by: Cassy Chun-Crogan <cassycc@google.com>
Commit-Queue: Ben Reich <benreich@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1265472}