FspCache: Evict multiple items at once

Multiple items need to be evicted (and removed) together when:
- The cache is downsizing.
- Notify() is called with multiple changed/deleted items.

Update EvictItems() (previously EvictContext()) to accept multiple items
to evict.

There is no logic change in this CL as items are still marked as evicted
and removed separately. However, in a follow up CL, items will also be
removed (if possible) in EvictItems(). This CL makes it easier to
facilitate eviction/removal in a single call.

Also update CacheFileContext.pending_removal to CacheFileContext.evicted
and rename EvictItems() to EvictExcessItems() as these are more
accurate.

Bug: b/337122469
Change-Id: I00c5c8f4fbb20a7399445ce7539fb5431feff8bd
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5543419
Reviewed-by: Ben Reich <benreich@chromium.org>
Commit-Queue: Cassy Chun-Crogan <cassycc@google.com>
Cr-Commit-Position: refs/heads/main@{#1302307}