Update javadoc of isApkVeritySupported

fs-verity is a kernel feature that we first developed to protect APKs.
Since the rollout requires a kernel update, we introduced this API as a
way for apps (mostly stores) to decide whether they need to act
differently on the device on app install.

The API has then also been used to check "whether fs-verity is
supported" on a device in other subsystems.

Since we're pivoting from the original plan, stop mentioning "APK
verity" in the API document.

Bug: 277347701
Test: m
Change-Id: Ib95e5f59c2323ba66fa348698110ca52a149d98d