[Cache Encryption] Add encryption cache buildflag

Since all platforms except iOS and Windows (subsequent CL to force this backend on Windows) use the same simple cache backend and encryption will apply, change pref build to reflect that and add new buildflag.

Bug: 454275908
Change-Id: I5041fc7b9f1a9cf62932d30d08a925193e77da29
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/7076091
Commit-Queue: Haihan Chen <haihan@google.com>
Reviewed-by: Owen Min <zmin@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1534486}
NOKEYCHECK=True
GitOrigin-RevId: 86b94b7b89ebe988389419389a122bad3b80866d

• resources/templates/policy_definitions/Miscellaneous/CacheEncryptionEnabled.yaml[diff]
• test/data/pref_mapping/CacheEncryptionEnabled.json[diff]